Optical mandrel, optical-fiber assembly including an optical mandrel, and system for detecting an acoustic signal incident on an optical-fiber assembly

ABSTRACT

An embodiment of a system includes a light source, an optical assembly, and an electronic circuit. The light source (e.g., a laser) is configured to generate a source optical signal. The optical assembly is configured to direct the source optical signal into an end of an optical-fiber assembly that includes an optical fiber having a section wrapped multiple turns around a mandrel and including mandrel zones, and to receive, from the end of the optical-fiber assembly, a return optical signal. The electronic circuit is configured to select at least one mandrel zone in response to a component of the return optical signal from the at least one mandrel zone, and to detect an acoustic signal incident on the mandrel in response to the component of the return optical signal.

PRIORITY

This patent application is a continuation to U.S. application Ser. No.16/563,578 filed Sep. 6, 2019 which claims priority to U.S. ProvisionalApplication Ser. No. 62/728,031, which was filed 6 Sep. 2018, which istitled OPTICAL MANDREL, AND FIBER-OPTIC-SENSING SYSTEM THAT INCLUDES THEMANDREL, and both of which are incorporated by reference.

SUMMARY

This disclosure applies to a class of optical-fiber sensors that launchlight into one end of an optical-fiber assembly and use the lightreflected or scattered back from different locations or zones in thefiber to detect a disturbance and to determine where along the fiber thedisturbance occurs. A system is configured for sensing an acousticsignal incident on an optical-fiber assembly. For example, if the systemis ground based, then the acoustic signal may be generated by avibration caused by a walking human or animal or by a moving vehicle.

Applications for such a system include providing perimeter security fora ground-based location such as a nuclear power plant, monitoring oil,natural gas, and other types of wells, and detecting and localizingunauthorized crossings of a land border between two or morejurisdictions.

To improve the ability of such a system to sense an acoustic signal, anoptical-fiber assembly may include, in addition to an optical fiber,optical mandrels spaced apart along the optical fiber. The opticalmandrels may be “in line” with the optical fiber, meaning that theoptical fiber passes through, and is wound about a form of, eachmandrel.

Each mandrel is configured to amplify an acoustic signal incident on themandrel as compared to an acoustic signal incident on a linear span(e.g., a span not located on a mandrel) of the optical fiber. Forexample, a mandrel may include a coating or covering, such as shrinkwrap, which increases the acoustic gain of the mandrel.

In further detail, a mandrel acts as a microphone with sensitivity thatis much higher than a typical zone (e.g., a one-meter long zone) of alinear span, or segment, of fiber. An acoustic disturbance can distort afiber segment directly, causing optical phase shifts, but the distortionis relatively small because the fiber is very stiff, so the resultingphase shifts are relatively small. A mandrel can be made of a materialthat is much more compliant than a glass optical fiber, so that anacoustic disturbance will distort the mandrel much more than it willdistort a segment of fiber; and the mandrel can impart this higherdistortion to a thin fiber because the fiber, being only part of themandrel, does not completely dominate the stiffness of the mandrelassembly. Effectively, the mandrel amplifies the distortion imparted tothe fiber by an acoustic disturbance. Furthermore, for acousticfrequencies low enough so that the wavelength of sound in thesurrounding medium is considerably longer than the mandrel's dimensions,all of the fiber wrapped around the mandrel will experienceapproximately the same compression or extension simultaneously. For thisreason, the phase changes across the individual mandrel zones (the zonesalong the segment of fiber wrapped around the mandrel) in the fiber addcoherently, creating a large phase change across the entire length ofthe fiber wound about the mandrel.

As described below, embodiments of a mandrel include one or morefeatures that improve the mandrel as compared to a conventional mandrel.

For example, the mandrel can include an optical-fiber path thattransitions an optical fiber from a linear span to a wound spangradually enough so that any bend along the optical-fiber path does notcause the optical-fiber to exhibit an undesirable characteristic in theregion of the bend.

Furthermore, the span of the optical fiber wound around the mandrel caninclude one or more reflectors to increase the signal-to-noise ratio(SNR), and the overall optical power, of an optical signal that arespective one or more zones of the wound optical-fiber span redirectback to signal-analysis circuitry.

In addition, the mandrel can be configured to allow an optical-fibersupport member to pass though the mandrel, thus eliminating the need tocut or remove the support member.

Also as described below, embodiments of the system include one or morefeatures that improve the system as compared to a conventional systemfor sensing an acoustic signal incident on an optical fiber.

For example, the system can be configured to select the “best”optical-fiber zones along a mandrel for detecting an incident acousticsignal. Further in example, the system can be configured to select thebest zones as the zones redirecting signals having the lowest noisecomponents or the highest optical powers. Still further in example, thesystem can be configured to select the two best zones near therespective ends of the mandrel, and to determine whether an acousticsignal is incident on the mandrel in response to respective componentsof a return optical signal redirected by the two selected zones.Alternatively, the system can be configured to select the best d zonesnear each of the respective ends of the mandrel, and to determinewhether an acoustic signal is incident on the mandrel in response to therespective components of a return optical signal redirected by the 2-dselected zones.

An embodiment of a mandrel includes a connector and a form. Theconnector has first and second aligned openings and a third openingbetween the first and second openings. And the form is coupled to theconnector at the third opening, has an outer surface, a cavity, an end,and fourth and fifth openings between the cavity and the outer surface,and is configured to receive an optical fiber that extends into thefirst opening of the connector, through the third opening of theconnector, into the cavity at the end of the form, and through thefourth opening of the form, that forms one or more turns around theouter surface of the form, and that extends through the fifth opening ofthe form into the cavity, out from the cavity at the end, into the thirdopening of the connector, and out from the second opening of theconnector.

Another embodiment of a mandrel includes an outer conduit, an innerconduit, and first and second end caps. The outer conduit has first andsecond ends, and the inner conduit is disposed inside of the outerconduit and has first and second ends. The first end cap has an outerend, an inner end coupled to the first end of the inner conduit andhaving a perimeter, and an optical-fiber path extending between theouter end and the perimeter, and the second end cap has an outer end, aninner end coupled to the second end of the inner conduit and having aperimeter, and an optical-fiber path extending between the outer end andthe perimeter.

An embodiment of an optical-fiber assembly includes an optical fiber andat least one mandrel each including a respective outer conduit havingfirst and second ends and about which a respective portion of theoptical fiber is wound, a respective inner conduit disposed inside ofthe outer conduit and having first and second ends, a respective firstend cap having an outer end, an inner end coupled to the first end ofthe inner conduit and having a perimeter, and an optical-fiber pathextending between the outer end and the perimeter and within which arespective portion of the optical fiber is disposed, and a respectivesecond end cap having an outer end, an inner end coupled to the secondend of the inner conduit and having a perimeter, and an optical-fiberpath extending between the outer end and the perimeter and within whicha respective portion of the optical fiber is disposed.

An embodiment of a system includes an optical fiber having an end, atleast one mandrel, and a signal detector. Each of the at least onemandrel includes a respective outer conduit having first and second endsand about which a respective portion of the optical fiber is wound, arespective inner conduit disposed inside of the outer conduit and havingfirst and second ends, a respective first end cap having an outer end,an inner end coupled to the first end of the inner conduit and having aperimeter, and an optical-fiber path extending between the outer end andthe perimeter and within which a respective portion of the optical fiberis disposed, and a respective second end cap having an outer end, aninner end coupled to the second end of the inner conduit and having aperimeter, and an optical-fiber path extending between the outer end andthe perimeter and within which a respective portion of the optical fiberis disposed. And the signal detector is configured to direct a sourceoptical beam into the end of the optical fiber, to receive a redirectedoptical beam from the end of the optical fiber, and to detect anacoustic signal incident on at least one of the at least one mandrel inresponse to the redirected optical beam.

Another embodiment of a system includes a light source, an opticalassembly, and an electronic circuit. The light source is configured togenerate a source optical signal. The optical assembly is configured todirect the source optical signal into an end of an optical-fiberassembly that includes an optical fiber having at least one section eachwrapped multiple turns around a respective one of at least one mandreland each including respective mandrel zones, and to receive, from theend of the optical-fiber assembly, a return optical signal. And theelectronic circuit is configured to select a first mandrel zone of oneof the at least one section of the optical fiber in response to a firstcomponent of the return optical signal redirected by the first mandrelzone, and to detect an acoustic signal incident on the one of the atleast one mandrel around which the one of the at least one section ofthe optical fiber is wound in response to the first component of thereturn optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an optical-sensor system that includes anoptical-fiber system with one or more optical mandrels, according to anembodiment.

FIG. 2 is a diagram, with portions broken away, of an optical mandrel,according to an embodiment.

FIG. 3 is a diagram of a portion of an optical assembly including anoptical fiber and an optical mandrel with a winding section aligned withthe optical fiber, according to an embodiment.

FIG. 4 is a diagram of the portion of the optical assembly of FIG. 3with the winding section orthogonal to the optical fiber, according toan embodiment.

FIGS. 5-6 are isometric views of a winding form of the winding sectionof the mandrel of FIGS. 3-4, and an optical fiber wrapped around thewinding form, according to an embodiment.

FIG. 7 is an isometric view of an optical mandrel, according to anotherembodiment.

FIG. 8 is an exploded view of the optical mandrel of FIG. 7, accordingto an embodiment.

FIG. 9 is a side view of the optical mandrel of FIGS. 7-8 with opticalfiber installed, according to an embodiment.

FIG. 10 is a cutaway side view of the optical mandrel of FIGS. 7-9,according to an embodiment.

FIG. 11 is an isometric view of an optical mandrel, according to yetanother embodiment.

FIG. 12 is an exploded view of the optical mandrel of FIG. 11, accordingto an embodiment.

FIG. 13 is a cutaway side view of the optical mandrel of FIGS. 11-12with optical fiber installed, according to an embodiment.

FIG. 14 is an isometric view of the optical mandrel of FIGS. 11-13 withportions transparent, according to an embodiment.

FIG. 15 is a graphical depiction of certain underlying physicalmechanisms of optical-signal polarizations.

FIG. 16 is a diagram of a system configured to detect an acoustic signalincident on one or more zones of an optical-fiber assembly, and of theoptical-fiber assembly, according to an embodiment.

FIG. 17 is a diagram of a balanced heterodyne optical detector circuit,according to an embodiment.

FIG. 18 is a diagram of a photodetector type heterodyner, according toan embodiment.

FIG. 19 is a diagram of a programmable correlator, which is configuredto enable spatial sampling of optical signals on mandrel and non-mandrelspans of an optical fiber that is part of the optical assembly of FIG.16 to provide a virtual array of acoustic-wave sensors along the opticalfiber, according to an embodiment.

FIG. 20 is a diagram of a set of phase-demodulator circuits configuredto receive outputs from the programmable correlator of FIG. 19,according to an embodiment.

FIG. 21 is a diagram of one of the phase-demodulator circuits of FIG.20, according to an embodiment.

FIG. 22 is a diagram of an IQ demodulator circuit of thephase-demodulator circuit of FIG. 21, according to an embodiment.

FIG. 23 is a diagram of a digital implementation of the phase-detectorcircuit of the phase-demodulator circuit of FIG. 21, according to anembodiment.

FIG. 24 is a diagram of an analog implementation of the phase-detectorcircuit of the phase-demodulator circuit of FIG. 21, according to anembodiment.

FIG. 25 is a diagram of a programmablerouting-and-phase-signal-switching network configured to provideselective pairings of the outputs of the set of phase demodulators ofFIG. 20 to provide differential phase signals across pairs of virtualacoustic-wave sensors along the optical fiber of the optical assembly ofFIG. 16, according to an embodiment.

DETAILED DESCRIPTION

“Approximately,” “substantially,” and similar words, as used herein,indicate that a given quantity b can be within a range b±10% of b, orb±1 if |10% of b|<1. “Approximately,” “substantially,” and similarwords, as used herein, also indicate that a range b-c can be fromb−0.10(c−b) to c+0.10(c−b). Regarding the planarity of a surface orother region, “approximately,” “substantially,” and similar words, asused herein, indicate that a difference in thickness between a highestpoint and a lowest point of the surface/region does not exceed 0.20millimeters (mm).

FIG. 1 is a diagram of an acoustic-signal detection system 1000, whichincludes an electro-optic sensor subsystem 1002, and which includes anoptical-fiber assembly 1004 having one or more optical mandrels 1006 andone or more non-mandrel spans 1008 of an optical fiber 1010, accordingto an embodiment. For example, the linear length (the length along alongitudinal axis) of the optical fiber 1010 can range from a few metersto tens of kilometers (km), the lengths of the mandrels 1006 can rangefrom about six inches to about twenty inches, and the linear lengths ofthe non-mandrel spans 1008 can range from about one meter to tens orhundreds of meters. Although the mandrels 1006 can have differentlengths, the mandrels each typically have a uniform length (e.g., twelveinches); similarly, although the non-mandrel spans 1008 can havedifferent linear lengths, the non-mandrel spans each typically have auniform linear length (e.g., fifty meters). Furthermore, the non-mandrelspans 1008 of the optical fiber 1010 each include one or more sections,hereinafter non-mandrel sensor zones, or non-mandrel zones, 1012, andthe spans of the optical fiber disposed on the mandrels 1006 eachinclude one or more mandrel zones 1014. Each non-mandrel zone 1012 andeach mandrel zone 1014 of the optical fiber encompasses a respectivelinear length, e.g., 1.0 m, of the optical fiber. Although the zones1012 and 1014 typically each have a same, uniform length, one mayconfigure the sensor subsystem 1002 to combine zones into sensingregions each having a length that is an integer multiple of the uniformzone length. For example, if the zones 1012 and 1014 each have a uniformlength of one meter, then one can configure the subsystem 1002 tocombine ten contiguous zones into a sensing region that has a length often meters. Furthermore, one can configure the subsystem 1002 to combineonly non-mandrel zones 1012 or only mandrel zones 1014 into such asensing region, or one can configure the subsystem to combinenon-mandrel zones and mandrel zones into a same sensing region.Furthermore, the uniform linear lengths of the zones 1012 and 1014 aretypically not fixed, and typically are configurable by configuringcertain operational characteristics of the electro-optic sensorsubsystem 1002, as explained below.

The sensor subsystem 1002 is configured to detect a vibration,hereinafter an acoustic signal, incident on one or more of thenon-mandrel and mandrel zones 1012 and 1014, and to determine one ormore of the following: on which zone(s) the detected acoustic signal isincident, parameters (e.g., amplitude, frequency, and phase) of thedetected acoustic signal, a location of a source of the detectedacoustic signal, and a classification of the source of the acousticsignal. An example of a classification is that the source of theacoustic signal is a vehicle moving at thirty miles per hour (mph), or ahuman walking at four feet per second.

Still referring to FIG. 1, operation of the system 1000 is described,according to an embodiment.

The sensor subsystem 1002 generates a source optical signal, such as amodulated laser beam, and directs the optical signal into the opticalfiber 1010.

As the optical signal propagates along the optical fiber 1010, eachregion, which has a tiny linear length, e.g., on the order ofnanometers, redirects a respective portion, or component, of the sourceoptical signal back along the optical fiber toward the sensor subsystem1002. For example, a mechanism that causes this redirection, also calledbackscattering, is the well-known mechanism of Rayleigh backscattering.

To reduce, to a suitable, or to an otherwise practical, value, thenumber of redirected components of the source optical signal that thesensor subsystem 1002 processes, the sensor subsystem uses coding,sampling, and correlation techniques to isolate, to monitor, and toanalyze only redirected components generated by zones of the opticalfiber 1010 that typically are each significantly longer than theabove-described regions of the optical fiber. For example, each fiberzone may be in the approximate range of one half to ten meters long in alinear length along the optical fiber 1010. Although the fiber zonestypically have a common length, one or more of the fiber zones can havea different length than one or more of the other fiber zones.Furthermore, using these same techniques, the sensor subsystem 1002 canselect which of the one or more fiber zones to monitor at any giventime, and can dynamically change the lengths of the fiber zones to varythe acoustic-signal-detecting resolution. Some of the coding, sampling,correlation, and other techniques that the sensor subsystem 1002 canimplement are described below in conjunction with FIGS. 15-25.

The optical fiber 1010 superimposes the redirected components of thesource optical signal (also “redirected components”) on one another toform a redirected optical signal that propagates along the optical fiberback toward the sensor subsystem 1002; that is, the redirected opticalsignal propagates in a direction that is opposite to the direction ofthe propagation of the source optical signal.

Assuming that no acoustic signal is incident on the optical fiber 1010,each redirected component has a different phase than the redirectedcomponents from the fiber zones near the fiber zone that generated theredirected component. That is, there is a phase difference, or phasedelta (also called “delta phase”), between the phases of two redirectedcomponents from immediately adjacent fiber zones.

In the absence of an acoustic signal incident on the optical fiber 1010,the phase deltas between respective pairs of fiber zones remainrelatively constant.

An acoustic signal incident on a zone of the optical fiber 1010 causesthat zone of the optical fiber to vibrate, and, therefore, to experiencestress (e.g., compressive or extensive force) and strain (e.g.,compressive or extensive length change).

The vibration-induced stress and strain causes the redirected componentof the source optical signal generated by the fiber zone and all zonesbeyond the fiber zone to have a phase alteration that is related to, andthat varies with, the amplitude of the incident acoustic signal. Forexample, if the incident acoustic signal is sinusoidal, then the phaseof the redirected component from the fiber zone also variessinusoidally.

Therefore, by determining and monitoring, over time, the phase of theredirected component from the zone of the optical fiber 1010 on whichthe acoustic signal is incident, the sensor subsystem 1002 determinesthe wave form of the acoustic signal, the wave form being proportionalto the time-dependent optical phase change.

Furthermore, by determining a phase difference between the redirectedcomponents from two zones along the optical fiber 1010, the sensorsubsystem 1002 can determine the acoustical disturbance on the region ofthe fiber between the two zones, permitting the location of thedisturbance to be determined. As indicated above, in the absence of anincident acoustic signal, the phase difference between first and secondredirected components from first and second adjacent zones of theoptical fiber 1010 is approximately constant. Furthermore, if theacoustic signal is incident on a zone of the optical fiber 1010 that ispast the first and second zones in a direction in which the sourceoptical signal propagates along the fiber, then the phases of the firstand second redirected components are, at least ideally, unchanged, sothat the phase difference between the first and second redirectedcomponents remains constant. Similarly, if the acoustic signal isincident on a zone of the optical fiber 1010 that is before the firstand second zones, then although the phases of the first and secondredirected components are changed because the phase change imparted tothe source optical signal by the acoustic signal propagates down thefiber to the first and second fiber zones, the phases of the first andsecond redirected components are, at least ideally, changed by a sameamount, so that the phase difference between the first and secondredirected components still remains constant. But if the acoustic signalis incident on the first zone of the optical fiber 1010 but not on thesecond zone of the optical fiber, or if the amplitude of the acousticsignal at the first zone is significantly different than the amplitudeof the acoustic signal at the second zone, then the acoustic signalimparts different optical phase changes to the first and secondredirected components, such that the phase difference between the firstand second redirected components is no longer constant, i.e., the phasedifference varies with respect to time. Therefore, the sensor subsystem1002 detecting a non-constant phase difference between two zones alongthe optical fiber 1010 indicates that an acoustic signal is incident atsome point of the optical fiber at, or between, the two zones. Forexample, if the sensor subsystem 1002 detects, over time, a non-constantphase difference between two one-meter-long zones that are immediatelyadjacent to one another in a non-mandrel span 1008 of the optical fiber1010, then this indicates that an acoustic signal is incident on thefiber at one of the two zones, thus providing anacoustic-signal-location resolution of two meters. Or if the sensorsubsystem 1002 detects, over time, a non-constant phase differencebetween two one-meter-long zones that are spaced apart by ten otherone-meter-long zones in a non-mandrel span 1008 of the optical fiber1010, then this indicates that an acoustic signal is incident to thefiber at one of the two zones, or at some location between the twozones, thus providing an acoustic-signal-location resolution of twelvemeters.

FIG. 2 is a cutaway side view, with portions broken away, of a portionof a mandrel 1006 of FIG. 1, according to an embodiment.

As described below, the mandrel 1006 can be configured to do one or moreof the following: amplify an acoustic signal incident on the mandrel,increase the acoustic-signal-location-determination resolution of thesystem 1000 (FIG. 1), and increase the signal-to-noise ratio (SNR) ofcomponents of the source optical signal redirected by at least some ofthe fiber zones along the mandrel.

The mandrel 1006 includes a hollow form 2000 about which is wound amandrel span 2002 of the optical fiber 1010; one may consider themandrel span of the optical fiber to be a part of the mandrel, or to beseparate from the mandrel. An interior, or cavity, 2004 of the form 2000can be filled with air, another gas, or a liquid depending on theapplication in which the mandrel 1006 is to be used. The form 2000 canbe configured, and can be filled with a fluid, so as to amplify anacoustic signal incident on the mandrel 1006 and, therefore, to providea stronger signal for the sensor subsystem 1002 (FIG. 1) to detect andto analyze; the mandrel 1006 may also include a covering, such as shrinkwrap, over the fiber 1010 and the form to protect the fiber and parts ofthe mandrel such as the form, and to further amplify an acoustic signalincident on the mandrel. Furthermore, the cavity 2004 can be configuredto allow an optional fiber support member 2006 to pass through so thatthe support member need not be cut at the mandrel. The support member2006 is, for example, a steel cable about which the optical fiber 1010is wrapped or otherwise attached or secured, and is configured to absorbstress, strain, and other forces during installation of theoptical-fiber assembly 1004 (FIG. 1) to prevent such forces fromsnapping, or otherwise breaking, the fiber. Alternatively, the opticalfiber 1010 can be disposed inside a protective coating or covering (notshown in FIG. 2) made from, e.g., rubber, along with fibrous strands(not shown in FIG. 2) of, e.g., Kevlar®, Nylon®, or plastic-reinforcedfiber glass that is wrapped, or that is otherwise disposed, around theoptical fiber; it is these fibrous strands that form the support member2006. Furthermore, the optical fiber may be coated with, e.g., a polymer(coating not shown in FIG. 2), which alone (e.g., in the absence of thefibrous strands) may form the support member 2006, or the combination ofthe coating and the fibrous strands may form the support member.

The mandrel 1006 increases the acoustic-signal-location-determinationresolution by effectively increasing the number of optical-fiber zonesin a given length L along the optical-fiber assembly 1004 (FIG. 1).

Referring to FIG. 1, in a non-mandrel span 1008 of the optical fiber1010, there is typically one non-mandrel zone per a given linear lengthof the optical fiber. For example, if a zone is one meter of the opticalfiber 1010 in a non-mandrel span 1008, then there is one non-mandrelzone per linear one meter of the optical fiber, and, therefore, perlinear one meter of the optical-fiber assembly 1004.

But, referring to FIG. 2, because the mandrel optical span 2002 of thefiber 1010 is wound around the form 2000, the mandrel 1006 effectivelyincreases the sensitivity of the of fiber zones within the mandreloptical span. As described above, the mandrel 1006 acts as a microphonewith sensitivity that is much higher than a typical zone (e.g., aone-meter long zone) of a non-mandrel span of the fiber 1010. Anacoustic disturbance can distort a fiber span directly, causing opticalphase shifts, but the distortion is relatively small because the fiberis very stiff, so the resulting phase shifts are relatively small. Butthe mandrel 1006 is made of a material that is much more compliant thanthe optical fiber 1010, so that an acoustic disturbance will distort themandrel much more than it will distort a segment of the fiber; and themandrel can impart this higher distortion to the relatively thin fiberbecause the fiber, being only part of the mandrel, does not completelydominate the stiffness of the mandrel. Effectively, the mandrel 1006amplifies the distortion imparted to the mandrel optical span 2002 ofthe fiber 1010 by an acoustic disturbance. Furthermore, for acousticfrequencies low enough so that the wavelength of sound in the medium(e.g., air or another gas, soil, water or another liquid) isconsiderably longer than the mandrel's dimensions, all of the fiber 1010wrapped around the mandrel 1006 experiences approximately the samecompression or extension simultaneously. For this reason, the phasechanges across the individual mandrel zones (the zones along the span2002 of fiber 1010 wrapped around the mandrel 1006) in the fiber addcoherently, creating, across the entire linear length of the fiber woundabout the mandrel, a significantly larger phase change than the acousticsignal would cause in the same length of fiber in a non-mandrel span1008.

For example, if the electro-optic sensor subsystem 1002 (FIG. 1) detectsa non-constant phase difference between redirected components of thesource optical signal from two zones of the optical fiber 1010 at eachend of the span 2002, and, therefore, at each end of the mandrel 1006,then this indicates that an acoustic signal is incident on the mandrel.

Because, in the above example, L=0.33 m, the mandrel provides anacoustic-signal-location-determination resolution of one third of ameter, which might be on the order of up to six times higher than theresolution provided by a non-mandrel span 1008 (FIG. 1) of optical fiber1010. That is, in the linear sections of the optical-fiber assembly 1004(FIG. 1) occupied by mandrels 1006, the optical-fiber assembly has anacoustic-signal-location-determination resolution that is higher thanthe resolution of the non-mandrel spans 1008.

Consequently, by selecting a suitable linear spacing of the mandrels1006 along the optical fiber 1010, one can design the system 1000 ofFIG. 1 to provide an acoustic-signal-location-determination resolutionthat is suitable for virtually any application. For example, in anembodiment of the system 1000 in which the optical-fiber assembly 1004is between five and ten kilometers long, immediately adjacent mandrels1006 can be spaced apart by a distance in an approximate range of tenmeters to one hundred fifty meters, for example fifty meters.

Referring to FIGS. 1-2, operation of the system 1000 is described,according to embodiments in which the system determines a phasedifference between different mandrel zones, or between differentsections, of a mandrel 1006.

In an embodiment, the sensory subsystem 1002 selects first and secondmandrel zones located in first and second halves, respectively, of themandrel portion 2008.

The sensory subsystem 1002 selects, as the first mandrel zone, the zonegenerating a redirected component of the source optical signal havingthe highest optical power as compared to the optical powers of theredirected components generated by the other zones in the first half ofthe mandrel portion 2008; for example, if ninety zones of the opticalfiber 1010 are wrapped around the portion 2008 of the mandrel form 2000,then each half of the mandrel portion 2008 includes forty five mandrelzones. The sensor subsystem 1002 can measure, or otherwise determine,the optical powers of the redirected components of the source opticalsignal using a conventional technique.

Similarly, the sensory subsystem 1002 selects, as the second mandrelzone, the zone generating a redirected component of the source opticalsignal having the highest optical power as compared to the opticalpowers of the redirected components generated by the other zones in thesecond half of the mandrel portion 2008.

In another embodiment, the sensor subsystem 1002 selects, as the firstmandrel zone, the zone generating a redirected component of the sourceoptical signal having the lowest noise content as compared to the noisecontents of the redirected components generated by the other zones inthe first half of the mandrel portion 2008; for example, if ninety zonesof the optical fiber 1010 are wrapped around the portion 2008 of themandrel form 2000, then each half of the mandrel portion 2008 includesforty five mandrel zones. The sensor subsystem 1002 can measure, orotherwise determine, the noise contents of the redirected components ofthe source optical signal using a conventional technique, or in thefollowing manner. The sensor subsystem 1002 applies a Fast FourierTransform (FFT) to the optical phase of each redirected component of thesource optical signal from the mandrel zones in the first half of themandrel portion 2008, and estimates the noise content of the redirectedcomponent as the standard deviation of the powers or energies of thefrequencies (binned by the FFT) in the upper half of the frequencyspectrum that the sensor subsystem is configured to analyze. The numberof frequencies available for this power or energy analysis may belimited by the frequencies that the FFT outputs; for example, a 64-pointFFT generates powers or energies for only sixty-four frequencies, thefundamental frequency and sixty-three harmonics of the fundamentalfrequency. For example, if the optical phase of the redirected opticalsignal is sampled at approximately 12 Khz (sampling, demodulation, andother processing of the redirected optical signal are described below inconjunction with FIGS. 15-25), then 0-6 Khz (i.e., the Nyquistfrequency) is the frequency spectrum of an acoustic signal that thesensor subsystem 1002 is configured to analyze. The sensor subsystem1002, therefore, identifies the noise content of the redirectedcomponent of the source optical signal from the mandrel zone as thestandard deviation of the acoustic power or energy in the phase of theredirected component at frequencies (e.g., at the frequencies that theFFT bins out outputs) within the range of 3 Khz-6 Khz, which is theupper half of a 0-6 Khz frequency spectrum. The lower the calculatedstandard deviation, the lower the determined noise content of thecorresponding redirected component, and the higher the calculatedstandard deviation, the higher the determined noise content of thecorresponding redirected component.

Similarly, the sensor subsystem 1002 selects, as the second mandrelzone, the zone generating a redirected component of the source opticalsignal having the lowest noise content as compared to the noise contentsof the redirected components generated by the other zones in the secondhalf of the mandrel portion 2008.

Still referring to FIGS. 1-2, operation of the system 1000 is described,according to embodiments in which the system determines an average phasedifference, or a difference between summed phase difference, betweenmore than two mandrel zones in different sections of a mandrel 1006.

In an embodiment, the sensory subsystem 1002 selects a number n of firstmandrel zones located in a first half of the mandrel portion 2008, andselects a number m of second mandrel zones located in a second half ofthe mandrel portion. Typically, n=m although this is not required.

The sensory subsystem 1002 can select the number n of first mandrelzones as the n mandrel zones in the first half of the mandrel portion2008 that generate redirected components of the source optical signalhaving the lowest noise contents or the highest optical-signal powers ascompared to the redirected components generated by the other mandrelzones in the first half of the mandrel portion.

Similarly, the sensor subsystem 1002 can select the number m of secondmandrel zones as the m mandrel zones in the second half of the mandrelportion 2008 that generate redirected components of the source opticalsignal having the lowest noise contents or the highest optical-signalpowers as compared to the redirected components generated by the othermandrel zones in the second half of the mandrel portion.

Next, the sensor subsystem 1002 determines the phases of the redirectedcomponents of the source optical signal from the selected n firstmandrel zones, and averages these phases to generate an average firstphase.

Similarly, the sensor subsystem 1002 determines the phases of theredirected components of the source optical signal from the selected msecond mandrel zones, and averages these phases to generate an averagesecond phase.

Then, the sensory subsystem 1002 determines the difference between thefirst and second average phases, and analyzes this phase difference perabove to determine whether an acoustic signal is incident on the mandrel1006.

In a related embodiment where n=m, the sensor subsystem 1002 determines,and sums together, the phases of the redirected components of the sourceoptical signal from the selected n first mandrel zones to generate atotal first phase.

Similarly, the sensor subsystem 1002 determines, and sums together, thephases of the redirected components of the source optical signal fromthe selected m second mandrel zones to generate a total second phase.

Then, the sensor subsystem 1002 determines the difference between thefirst and second total phases, and analyzes this phase difference perabove to determine whether an acoustic signal is incident on the mandrel1006.

Still referring to FIGS. 1-2, in a variation of the above-describedembodiments, the sensor subsystem 1002 partitions the mandrel portion2008 into other than halves for the above-described phase-differencingtechniques. For example, the sensor subsystem 1002 can partition themandrel portion 2008 intop partitions, and select two mandrel zones forphase differencing, one from each of the end partitions, using one ofthe above-described lowest-noise and highest-optical-power techniques.Further in example, where p=6 and the number of mandrel zones in thespan 2002 is ninety, then the sensor subsystem can be configured toselect one mandrel zone from the 90/6=15 mandrel zones at one end of themandrel portion 2008, and to select another mandrel zone from thefifteen mandrel zones at the other end of the mandrel portion.

In yet another variation of the above-described embodiments, the sensorsubsystem 1002 partitions the mandrel portion 2008 into other thanhalves for the above-described phase-average-differencing andphase-sum-differencing techniques. For example, the sensor subsystem1002 can partition the mandrel portion 2008 into p partitions, select nmandrel zones for phase-averaging or phase-sum differencing from one ofthe end partitions, and select m mandrel zones for phase-averaging orphase-sum differencing (m=n for phase-sum differencing) from the otherend partition, using one of the above-described lowest-noise andhighest-optical-power techniques. Further in example, where p=6 and thenumber of mandrel zones in the span 2002 is ninety, then the sensorsubsystem can be configured to select n mandrel zones from the 90/6=15mandrel zones at one end of the mandrel portion 2008, and to select mmandrel zones from the fifteen mandrel zones at the other end of themandrel portion. In general, the methods exemplified here, for computingan effective optical phase change for the fiber on the mandrel, can bedescribed as computing a weighted sum of the phase changes of selectedzones in and near the mandrel, where the weights can be both positiveand negative. The zones and their weights can be chosen in differentways that optimize the signal-to-noise ratio.

Referring to FIG. 2, in still another embodiment of the mandrel 1006,enhanced redirectors 2010 (for example, enhanced reflectors), can beconventionally formed in respective portions of the optical fiber 1010forming the two end mandrel zones 1014 to increase the redirection gainof the end mandrel zones, and, therefore, to increase the optical powerand optical-signal amplitudes of the components of the source opticalsignal respectively redirected by the end mandrel zones. That is, theredirectors 2010 are configured to increase the SNR of the redirectedcomponents from the end mandrel zones. The sensor subsystem 1002(FIG. 1) can be configured, a priori, with the “knowledge” of the endmandrel zones having the redirectors 2010 such that the sensor subsystemis configured to use only the redirected components of the sourceoptical signal for phase differencing and other phase analysis.Alternatively, the sensor subsystem 1002 can implement one or more ofthe above-described phase-differencing techniques, where it isunderstood that most or all of the time, the two end mandrel zonesincluding the redirectors 2010 generate the redirected components of thesource optical signal having the lowest noise contents or the highestoptical powers.

Still referring to FIG. 2, in more detail, where the sensor subsystem1002 (FIG. 1) uses redirected components of the source optical signaldue to Rayleigh scattering from only a small number of the zones in theoptical fiber 1010 wound about, or located just outside of, the mandrelportion 2008, (e.g. at or near the ends of a mandrel 1006), there aremany other mandrel and non-mandrel zones in the fiber producing Rayleighbackscatter light that the sensor subsystem 1002 effectively ignores.Certain imperfections in the optical fiber 1010 or sensor subsystem 1002(e.g., clock jitter, laser phase noise, or phase shifts due todisturbance of the optical fiber) can cause crosstalk from these ignoredzones into the zones generating the redirected components of the sourceoptical signal that the sensor subsystem 1002 processes, resulting inundesirable noise in these latter zones. To mitigate this noise, it ispossible to place either discrete reflectors or high-reflection orhigh-scattering zones (called here “redirectors”) in the fiber near thetwo ends of the mandrel 1006, in which the redirected (e.g., reflectedor scattered) optical power is considerably higher than the typicaloptical power from ignored, or other, mandrel and non-mandrel zones thathave only Rayleigh scattering. Therefore, crosstalk from such Rayleighscattering from these ignored and other zones has a much smaller effecton the redirected components from the used zones that include theredirectors 2010, creating less noise. Furthermore, the redirectors 2010can be made, for example, with splices containing materials added to theglass of the optical fiber 1010, with Bragg gratings, with fiber-opticconnectors that have some reflection, or with segments of fiber withscattering or reflection enhanced by other means, which may be builtinto the fiber or added by splicing a special segment into the fiber.The redirection gain of such a reflector zone (a zone including aredirector 2010) is typically substantially more than the typicalRayleigh backscattering from an average zone (e.g., a zone non includinga redirector 2010) in the fiber, for example, by a factor of four ormore, further in example by a factor of one hundred or more. In someapplications the redirection coefficient of a redirector 2010 might evenbe in an approximate range as high as 1% to 10% of the optical power ofthe incident source optical beam. Alternatively, in a system 1000 (FIG.10) with many (e.g., ten or more) mandrels, it might be desirable tomake the reflection coefficient of each redirector 2010 within anapproximate range of 0.01% to 0.1%.

Referring to FIGS. 1-2, a summary of the operation of the system 1002using mandrels is presented. Details of at least some of the conceptsincluded in this summary are described below in conjunction with FIGS.15-25.

Mandrel Acoustic Signal Formation

In an embodiment, the electro-optic sensor subsystem 1002 effectivelyforms an acoustic signal for a mandrel 1006 by selecting a number w,e.g., w=15, best mandrel zones 1014, also called mandrel sensors, in thebeginning of the mandrel, and a same number w of the best mandrelsensors at the end of the mandrel. The sensor subsystem 1002 determinesthe w best mandrel sensors 1014 from the beginning of the mandrel 1006and the w best sensors from the end of the mandrel by analysis of eachsensor's high-frequency noise or by other methods such as optical-poweranalysis.

In more detail, it is useful to divide time into discrete intervalscalled “measurement intervals,” and to divide the fiber 1010 intodiscrete regions called “zones” or “sensors.” For example, in anembodiment, successive zones might be approximately one meter long,corresponding to a round-trip time increment of approximately tennanoseconds for light (a modulated source optical signal) in the fiber;and a measurement interval might be 81.91 microseconds (s) long,corresponding to the round-trip delay for a fiber that is 8,191 zoneslong, and corresponding to the number of bits in a particularpseudo-random code sequence. With the data produced by the sensorsubsystem 1002, the sensor subsystem can compute optical phaseinformation for each zone and for each measurement interval. This phaseinformation can be the effective “absolute optical phase” of thereturning light (redirected optical signal) from the zone, relative tothe optical phase of the optical local-oscillator signal; or this phaseinformation can be the “phase change” between the current measurementinterval and a previous measurement interval (e.g., the immediatelyprior measurement interval), i.e., the change, or delta, in absoluteoptical phase. Whichever is used (absolute optical phase or phase changeover time), this zone-dependent signal is, at least for purposes of thissection, called the “signal.”

A disturbance in a section of the fiber 1010 that is close to the launchend of the fiber (the end coupled to the electro-optic sensor subsystem1002) creates a signal in all of the fiber zones, both non-mandrel zones1012 and mandrel zones 1014, that are “downstream,” or more distant fromthat zone. Therefore, signals measured for a particular zone may not berepresentative of fiber disturbances in the vicinity of that zone. Forthis reason, it is useful to compute the “delta signal,” meaning thedifference between the signal values for two zones. The two zones may becontiguous with each other, or may be at some distance from each other,but in either case the measured difference is representative ofdisturbances in the section of the fiber 1010 that is between the twozones.

A mandrel 1006 is designed to subject all mandrel zones 1014 (the zonesin the section of the fiber 1010 wound about the mandrel) toapproximately the same acoustic disturbance, so that the phase effectsaccumulate coherently from one end of the mandrel to the other end. Thismakes the delta signal between a mandrel zone 1014 at the start of themandrel 1006 and one at the end of the mandrel relatively large. Butsome zones produce “poor” signals that are noisy, primarily because thestatistical nature of Rayleigh scattering can cause the total opticalpower scattered back from a particular zone to be low. So instead ofjust looking at the delta signal between the mandrel zones 1014 atprecisely the two ends of the mandrel 1006 (which delta signal might bepoor because one or both of the mandrel zones is/are poor), the sensorsubsystem 1002 can be configured (1) to choose, near each end of themandrel, a respective mandrel zone that is especially “good” in thesense of having relatively low noise or relatively high optical power orother attributes. Furthermore, instead of just using single mandrelzones, one at each end, the sensor subsystem 1002 can be configured (2)to use several good mandrel zones at each end of the mandrel and averagethem (e.g., average the phases, amplitudes, or polarizations of thebackscattered signals from the zones), since they will haveapproximately the same signal and such averaging will generally reducenoise.

For these reasons, in an embodiment, the electro-optic sensor subsystem1002 effectively forms an acoustic signal for a mandrel 1006 byselecting a number of the best mandrel sensors (such as w=15) in thebeginning of the mandrel and the same number w, or a similar number(e.g., w−1) of the best mandrel sensors from the end of the mandrel. Thesensor subsystem 1002 can be configured to determine the best mandrelsensors from the beginning and the end of the mandrel by analysis ofeach sensor's high-frequency noise or other methods such asoptical-power analysis, such as computing the sum of the squares of thein-phase and quadrature components from the IQ demodulator (see, e.g.,FIG. 22). Once the sensor subsystem 1002 identifies the best mandrelsensors (mandrel zones 1014), the sensor subsystem sums, e.g., thephases of the chosen mandrel sensors at the beginning of the mandrelinto a variable sum1; similarly, the sensor subsystem 1002 sums, e.g.,the signal phases, from the chosen mandrel sensors at the end of themandrel to form sum2. Next, the sensor subsystem 1002 computes thedifference between sum2 and sum1 to represent the “delta signal,” or the“delta acoustic signal,” between the two ends of the mandrel 1006.

AutoEncoder

The sensor subsystem 1002 can include, or can implement, amachine-learning algorithm to provide a target-detection circuit andalgorithm. Machine-learning, or deep-learning, algorithms such as anautoencoder, recurrent neural network, convolutional neural network(CNN), or other neural-network architecture could be used for thispurpose. By detection, it is meant the determination of whether an eventsensed by the fiber 1010 is statistically significant.

Operationally, the autoencoder is trained using normal operatingenvironmental data from the mandrels 1006, and it learns the backgroundnoise signature for each mandrel. When the system 1000 is running andcollecting acoustic data, the autoencoder processes “chunks” (e.g.,three-second segments) of audio data and reconstructs the acousticsignal using the autoencoder model. The autoencoder, or another part ofthe subsystem 1002, computes the root-mean-square error between theacoustic signal and the autoencoder's reconstructed signal. If theroot-mean-square error exceeds a predefined threshold, the sensorsubsystem 1002 determines the acoustic signal to be anomalous. Exceedingthe pre-defined root-mean-square-error threshold defines the detectionof an acoustic signal of interest. When an anomalous acoustic signal isdetected by the autoencoder, the anomalous acoustic signal (e.g., thechunk of the signal) is further processed by a previously trained CNN todetermine the target type. That is, the CNN classifies the acousticsignal as, e.g., human activity (walking, running), vehicle, aircraft,drone or background target types.

Still referring to FIGS. 1-2, alternate embodiments of the system 1000are contemplated. For example, the optical-fiber assembly 1004 canterminate in a non-mandrel span 1008 instead of in a mandrel 1006.Furthermore, embodiments described in conjunction with FIGS. 3-25 may beapplicable to the system 1000 of FIG. 1 and the mandrel 1006 of FIG. 2.

FIG. 3 is a diagram of a portion 3000 of the optical-fiber assembly 1004of FIG. 1 including a portion of the optical fiber 1010 and an opticalmandrel 1006 with a winding section 3002 aligned with the optical fiber,according to an embodiment.

FIG. 4 is a diagram of the portion 3000 of the optical-fiber assembly1004 of FIG. 3 with the winding section 3002 of the mandrel 1006oriented orthogonal to the optical fiber 1010, according to anembodiment.

Referring to FIGS. 3-4, in addition to the winding section 3002, themandrel 1006 includes a T-connector 3004 having a first section 3006 anda second section 3008, which is orthogonal to the first section. Thefirst section 3006 includes first and second aligned openings 3010 and3012 via which the optical fiber 1010 enters and exits the mandrel 1006,and the second section 3008 includes a third opening 3014. A right-angleelbow joint 3016 couples the winding section 3002 to the third opening3014, and allows one to rotate the joint and winding section relative tothe second section 3008 in a plane perpendicular to the page of FIG. 3.Although not shown in FIGS. 3-4, the optical fiber 1010 enters one ofthe first and second openings 3010 and 3012, extends through the secondsection 3008, the third opening 3014, and the elbow joint 3016, to thewinding section 3002, and further extends from the winding section, backthrough the elbow joint, the third opening, and the second section,through the first section, and exits the other one of the first andsecond openings. The mandrel 1006 can also include flexible fiber grips3018 and 3020 for reinforcing the optical fiber 1010, and, therefore,preventing breaking of the optical fiber, at the first and secondopenings 3010 and 3012; the flexible fiber grips can be attached to thefirst section 3006 of the T-connector 3004 by engaging the first andsecond openings.

The winding section 3002, T-connector 3004, elbow join 3016, and fibergrips 3018 and 3020 of the mandrel 1006 each can be formed from anyrespective suitable material such as metal, plastic, or resin, and canhave any suitable sizes.

Furthermore, the winding section 3002 can be configured to amplifyacoustic signals incident on the winding section.

Moreover, the first, second, and third openings 3010, 3012, and 3014, aswell as the interface between the winding section 3002 and the elbow3016, can be fitted with sealing members (e.g., with O-rings) that formfluid-tight seals to prevent contaminants from entering inside themandrel 1006.

In addition, the mandrel 1006 is configured so that any bends that theoptical fiber 1010 experiences within, or entering or exiting, themandrel have bend radii large enough to prevent significant degradationof the optical-signal propagation characteristics of the optical fiber.

FIGS. 5-6 are isometric views of a winding body or form 5000, which islocated inside of the winding section 3002 of the mandrel 1006 of FIGS.3-4, and a portion of the optical fiber 1010 wrapped around the portion2008 of the winding form, according to an embodiment. The mandrel 1006also can include a covering (not shown in FIGS. 5-6) over the form 5000and the optical fiber 1010.

The form 5000 has a longitudinal axis 5001, can be made from anysuitable material, can have any suitable dimensions, and can have anysuitable cross-sectional shape. For example, the form 5000 can be madefrom a material (e.g., a polymer) that is sufficiently flexible so asnot to restrict the winding of the optical fiber 1010 from sensingvibrations, or that is configured to amplify a vibration by distributinga vibration directed to one or more points of the winding across theentire winding. Because the form 5000 is cylindrical, it does not impartsharp bends to the fiber 1010 wound about the portion 2008 so as tolimit bending signal losses in in the wound fiber (to further limitbending losses, an optical fiber with low bending loss can be used forthe optical fiber 1010). Furthermore, low- or zero-bend-loss fibers andfusion splicing can be used for the section of the fiber wound about themandrel portion 2008 to minimize back reflections as a result of fiberturns and splice loss. But the form 5000 can have another othercross-sectional shape, even a shape (e.g., square, triangular) that doesimpart sharp bends to the wound fiber 1010. Furthermore, the shorter theform 5000, the higher the resolution of vibration-source-locationdetermination that the form can provide, but the smaller thevibration-beam-forming aperture that the form can provide. Conversely,the longer the form 5000, the lower the vibration-source-locationdetermination that the form can provide, but the larger thevibration-beam-forming aperture that the form can provide. Moreover, oneor both ends of the form 5000 can be capped, e.g., with respective plugsof vulcanized rubber, and openings for the wound fiber 1010 to extendfrom the form can be formed in the end caps or in the form itself. Forexample, the cylindrical form 5000 can be made from any suitablematerial such as an acrylic or polyvinyl chloride (PVC), polycarbonate,carbon-fiber, or glass, can have an air core (e.g., an air-filledcavity), can have a length of approximately 12 inches (˜0.33 m), and canhave an outside diameter of approximately 1.6 inches. Additional detailsof an embodiment of the form 5000 are set forth below.

Instead of being formed from a length of the optical fiber 1010, thewound portion of the optical fiber also can be formed from a separatespan of optical fiber that is spliced to the optical fiber 1010 at ornear winding ends 5002 and 5004. If a separate optical fiber forms thefiber section wound about the portion 2008, then the separate opticalfiber can be the same as, or different than, the optical fiber 1010. Forexample, the separate optical fiber can be a low-bending-loss fiber asdescribed above, such as Corning ZBL or the AFL equivalent. Furthermore,each turn of the optical fiber wound about the form portion 2008 cancontact the one or two adjacent turns, or can be separated from the oneor two adjacent turns by a uniform or respective spacing (end turns areeach adjacent only to one other turn, intermediate turns are eachadjacent to two other turns, one other turn on each side of theintermediate turns); for example, the spacing between a pair of adjacentturns can be the same as, or different from, the spacing between anotherpair of adjacent turns. Moreover, the wound section of the fiber 1010can be unsecured relative to the form 5000, or can be secured with, forexample, an adhesive that bonds the fiber to the form. Alternatively,channels or grooves (not shown in FIGS. 5-6) can be formed in the form5000 to receive turns of the wound optical fiber 1010. In addition, thewound fiber 1010 can have any suitable number of partial or full turns.Furthermore, the wound section of the fiber 1010 can have only one, orcan have more than one, layer of turns. Moreover, the wound section ofthe fiber 1010 can occupy any portion of the length of the form 5000 upto the entire length of the form; that is, the portion 2008 of the formabout which the fiber is would can have any suitable length up to theentire length of the form. And any portion of the fiber 1010 inside ofthe form 5000 (e.g., so that the winding ends 5002 and 5004 can exit viathe ends of the form) can be secured to the inside of the form, e.g.,with an adhesive such as epoxy. In an embodiment, for a form 5000 havingan outside diameter of approximately 1.6 inches and a length ofapproximately 12 inches, the length of the optical fiber 1010 (or of aseparate span of fiber) that wraps around the form is approximate 96meters (approximately 315 feet), and the number of turns that the woundfiber makes about the form is approximately seven-hundred-fifty (750)turns. If each mandrel zone is approximately one meter in linear lengthalong the optical fiber 1010, then the mandrel 1006 includesapproximately 96 non-overlapping mandrel zones.

The covering (not shown in FIGS. 5-6) over the fiber 1010 and form 5000can be any suitable material, such as heat-shrink tubing oradhesive-backed heat-shrink tubing. The covering can protect the form5000, the section of the optical fiber wound about the form portion2008, and other portions of the mandrel 1006 from contaminants such asdirt and moisture, and can hold the turns of the optical fiber in place.Such protection can be sufficient to allow one to use the mandrel 1006in virtually any medium or substance, for example, to bury the mandrelunderground, and to embed the mandrel in a building material such asconcrete. Furthermore, the covering can have a flexibility sufficient toallow the optical fiber wound about the form 5000 to distort, and toexperiences force, in response to vibrations so that the system 1000(FIG. 1) can detect and classify the vibrations. In addition, thecovering can protect against moisture buildup inside of the mandrel 1006and can minimize damage to the wound section of the optical fiber thatmay occur during freeze/thaw cycles. Fiber internal to the form 5000 canbe secured to the inner wall of the form to prevent excessive movementof the fiber during and after deployment. The mandrel 1006 can befastened, or otherwise coupled, to the optical fiber 1010 external tothe mandrel in a manner that reduces the chances of fiber pull out andthat provides strain relief to wound fiber if the wound fiber is splicedto the optical fiber 1010.

Referring to FIG. 5, in an embodiment the mandrel 1006 includes an aircore, with a low-bend-loss single-mode fiber 1010 (or a separate span ofoptical fiber) wrapped around a thin-wall form 5000, which can be formedfrom a thin-wall acrylic, or thin-wall PVC, at an outside diameter(O.D.) of approximately 1.6 inches. The low-bend-loss fiber 1010 wrappedaround the form 5000 can be, for example, Corning ZBL or the AFLequivalent. The form 5000 is approximately 12 inches in length (thedimension along the axis 5001), with two bevel-edged holes 5008machined, one at each end of the form approximately 1.6 inches in fromthe respective end of the form. The wound section of the fiber 1010 iswrapped around the portion 2008 of the form 5000 with consistent tensionand spacing accomplished with a customized lathe/tensioner (not shown inFIGS. 5-6) to turn and advance the feed as more fiber is wrapped aroundthe form. This results in an approximately uniformly tensioned andspaced wrapping with each turn of the fiber 1010 touching the adjacentand previous turns with minimal deformation of the fiber geometry. Forexample, the fiber 1010 is wrapped around the form 5000seven-hundred-fifty-eight (758) times and is secured to the form asdescribed above, and then the mandrel 1006 removed from the lathe.

Still referring to FIG. 5, an end 5010 of the mandrel 1006 is set intothe third opening 3014 (FIGS. 3-4) of the T-coupler 3004 to allow fordirectionality for suitable detection and sensitivity, while an end 5012is capped with a vulcanized rubber seal. The wound fiber at the end 5010of the form 5000 is removed by three (3) turns and fed back through theadjacent opening 5008 into the cavity of the form 5000, exiting out theend 5010 into the T connector 3004 (FIGS. 3-4) to be prepared for fusionsplicing into the optical fiber 1010 if the span of fiber wrapped aroundthe form is not integral with the fiber 1010. Similarly, the wound fiberat the end 5012 of the form 5000 is removed by five turns and fed backthrough the adjacent opening 5008 into the cavity of the form, exitingout the end 5010 into the T connector 3014 to be prepared for fusionsplicing into the optical fiber 1010 if the fiber wrapped around theform 6000 is not integral with the fiber 1010 (e.g., the fiber externalto and internal to the mandrel 1006 is not a single piece).

The mandrel of FIG. 6 is similar except that the fiber 1010 (or separatesection of fiber spliced to the fiber 1010) enters one of the form 5000ends 5010 and 5012 and exits the other of the ends 5010 and 5012.

Referring to FIGS. 3-6, alternate embodiments of the mandrel 1006 arecontemplated. For example, one or more embodiments described inconjunction with FIGS. 1-2 and 7-25 may be applicable to the mandrel1006 of FIGS. 3-6.

FIG. 7 is an isometric view of an optical mandrel 1006 of FIG. 1,according to another embodiment.

FIG. 8 is an exploded view of the optical mandrel 1006 of FIG. 7,according to an embodiment.

Referring to FIGS. 7-8, as described below, the optical mandrel 1006 isconfigured to be disposed “in-line” with the optical fiber 1010 in theoptical-fiber assembly 1004 of FIG. 1, and includes optical-fiber paths(not visible in FIGS. 7-8) that are configured to bend the optical fibergradually enough to prevent the fiber bends from changing thepropagating and redirecting (e.g., backscattering) characteristics ofthe optical fiber as compared the redirecting characteristics of astraight span of the fiber. Furthermore, the optical mandrel 1006 isdesigned to amplify acoustic signals incident on the mandrel such thatthe acoustic signals that the electro-optic sensor subsystem 1002 ofFIG. 1 recovers from the redirected optical signal are more likely tohave a higher optical power and a higher SNR as compared to acousticsignals that the sensor subsystem 1002 recovers from non-mandrel zonesof the optical fiber 1010 of FIG. 1.

The optical mandrel 1006 includes first and second fiber grips 7000 and7002, first and second end caps 7004 and 7006, an inner conduit 7008,and outer conduit 7010, and first and second O-rings 7012 and 7014.

The first fiber grip 7000 has inner and outer ends 7016 and 7018 withrespective inner and outer openings 7020 (not visible in FIGS. 7-8) and7022 configured to receive an optical fiber (not shown in FIGS. 7-8), ismade from a flexible material such as a rubber and is otherwiseconfigured to be flexible, and is configured to prevent the opticalfiber from breaking, or from otherwise becoming damaged, as the fibertransitions between a non-mandrel span and the mandrel 1006; forexample, the first fiber grip 7000 can include spaced segments 7024,which increase the flexibility of the first fiber grip as compared to asegment-less fiber grip. Furthermore, the inner end 7016 is configuredfor attachment to the first end cap 7004; for example, the inner end caninclude male threads 7026 for engaging female threads (not visible inFIGS. 7-8) of the first end cap. Moreover, the inner and outer openings7020 and 7022 are configured to make a fluid-tight, or liquid-tight,seal with the optical fiber that extends through the first and secondopenings. In addition, the first fiber grip 700 can have a length ofapproximately 1.5-2.5 inches (e.g., 2.0 inches), an inner diameter ofapproximately 0.1-0.2 (e.g., 0.16 inches), and an outer diameter ofapproximately 0.6-1.0 inches (e.g., 0.8 inches).

Similarly, the second fiber grip 7002 has inner and outer ends 7028 and7030 with respective inner and outer openings 7032 (not visible in FIGS.7-8) and 7034 configured to receive an optical fiber (not shown in FIGS.7-8), is made from a flexible material such as a rubber and is otherwiseconfigured to be flexible, and is configured to prevent the opticalfiber from breaking, or from otherwise becoming damaged, as the fibertransitions between a non-mandrel span and the mandrel 1006; forexample, the second fiber grip 7002 can include spaced segments 7036,which increase the flexibility of the first fiber grip as compared to asegment-less fiber grip. Furthermore, the inner end 7028 is configuredfor attachment to the second end cap 7006; for example, the inner endcan include male threads 7038 for engaging female threads (not visiblein FIGS. 7-8) of the first end cap. Moreover, the inner and outeropenings 7032 and 7034 are configured to make a fluid-tight, orliquid-tight, seal with the optical fiber that extends through the firstand second openings. In addition, the second fiber grip 7002 can have alength of approximately 1.5-2.5 inches (e.g., 2.0 inches), an innerdiameter of approximately 0.1-0.2 (e.g., 0.16 inches), and an outerdiameter of approximately 0.6-1.0 inches (e.g., 0.8 inches).

The first end cap 7004 includes an inner end 7040, which includes aninner receptacle 7042 and an outer receptacle 7044, and includes anouter end 7046. The first end cap 7004 can be formed from any suitablematerial such as metal, plastic, or resin. The inner receptacle 7042 isconfigured for attachment to an end of the inner conduit 7008; forexample, the inner receptacle may include female threads configured toengage male threads on the end of the inner conduit. The outerreceptacle 7044 is configured to hold the O-ring 7012 and for receivingan end of the outer conduit 7010; for example, the outer receptacle canbe flanged so as to fit snugly around the end of the outer conduit,which, when installed, presses against the O-ring inside a back of theouter receptacle to provide a fluid-tight or liquid-tight seal thatprevents contaminants such as dirt and water from entering the innercavity formed by the outer conduit. The outer end 7046 is configured forreceiving and attaching to the inner end 7016 of the first fiber grip7000; for example, as described above, the outer end may include femalethreads configured for engaging male threads 7026 of the first fibergrip. In addition, the first end cap 7004 can have a length ofapproximately 2.0-3.0 inches (e.g., 2.5 inches) inner receptacle 7042can have a diameter of approximately 0.4-0.8 inches (e.g., 0.6 inches),the outer receptacle 7044 can have a diameter of approximately 1.3-1.7inches (e.g., 1.5 inches), and the outer end 7046 can have an innerdiameter approximately equal to the diameter of the inner receptacle andcan have an outer diameter of approximately 1.0-2.0 inches (e.g., 1.25inches).

Similarly, the second end cap 7006 includes an inner end 7050, whichincludes an inner receptacle 7052 and an outer receptacle 7054, andincludes an outer end 7056. The second end cap 7006 can be formed fromany suitable material such as metal, plastic, or resin. The innerreceptacle 7052 is configured for attachment to an end of the innerconduit 7008; for example, the inner receptacle may include femalethreads configured to engage male threads on the end of the innerconduit. The outer receptacle 7054 is configured to hold the O-ring 7014and for receiving an end of the outer conduit 7010; for example, theouter receptacle can be flanged so as to fit snugly around the end ofthe outer conduit, which, when installed, presses against the O-ringinside a back of the outer receptacle to provide a fluid-tight orliquid-tight seal that prevents contaminants such as dirt and water fromentering the inner cavity formed by the outer conduit. The outer end7056 is configured for receiving and attaching to the inner end 7028 ofthe second fiber grip 7002; for example, as described above, the outerend may include female threads configured for engaging male threads 7038of the second fiber grip. In addition, the second end cap 7006 can havea length of approximately 2.0-3.0 inches (e.g., 2.5 inches) innerreceptacle 7052 can have a diameter of approximately 0.4-0.8 inches(e.g., 0.6 inches), the outer receptacle 7054 can have a diameter ofapproximately 1.3-1.7 inches (e.g., 1.5 inches), and the outer end 7056can have an inner diameter approximately equal to the diameter of theinner receptacle and can have an outer diameter of approximately 1.0-2.0inches (e.g., 1.25 inches).

The inner conduit 7008 is a hollow pipe nipple that includes first andsecond ends 7060 and 7062. The inner conduit 7008 can be made from anysuitable material, such as metal or plastic. Each of the first andsecond ends 7060 and 7062 are configured for attachment to the innerreceptacles 7042 and 7052 of the first and second end caps 7004 and7006, respectively; for example, as described above, the first andsecond ends of the inner conduit 7008 can have male threads 7064 and7066 configured to engage female threads of the inner receptacles 7042and 7052, respectively. It is the inner conduit 7008 that couplestogether the first and second end caps 7004 and 7006, and, therefore,that “holds the mandrel 1006 together.” The first and second ends 7060and 7062 can be configured to make a fluid-tight or a liquid-tight sealwith the inner receptacles 7042 and 7052 of the first and second endcaps 7004 and 7006, respectively. Furthermore, the inner conduit 7008can have a length of approximately 8.0-10.0 inches (e.g., 8.8 inches),an outer diameter of approximately 0.6-0.8 inches (e.g., 0.7 inches),and an inner diameter of approximately 0.5-0.7 inches (e.g., 0.6inches).

The outer conduit 7010 is a cylinder or tube that includes first andsecond ends 7070 and 7072 and is the component of the mandrel 1006 aboutwhich an optical fiber (e.g., the optical fiber 1010 of FIG. 1) iswound. The outer conduit 7010 can be made from any suitable material,such as metal or plastic. And although shown as being transparent tolight, the outer conduit 7010 may be opaque. Each of the first andsecond ends 7070 and 7072 are configured for fitting into the outerreceptacles 7044 and 7054 of the first and second end caps 7004 and 7006and pressing against the O-rings 7012 and 7014, respectively. The firstand second ends 7070 and 7072, therefore, make a fluid-tight or aliquid-tight seal with the O-rings 7012 and 7014, respectively, toprevent contaminants such as water and dirt from entering the cavityformed by the outer conduit 7010. Furthermore, the outer conduit 7010can have a length of approximately 8.0-10.0 inches (e.g., 8.8 inches),an outer diameter of approximately 1.0-2.0 inches (e.g., 1.5 inches),and an inner diameter of approximately 0.9-1.9 inches (e.g., 1.4inches).

And the O-rings 7012 and 7014 can be conventional and made from anysuitable material such as a rubber. Furthermore, each of the O-rings7012 and 7014 can have inner and outer diameters that are approximatelythe same as the inner and outer diameter of the outer conduit 7010, thatis, an outer of approximately 1.0-2.0 inches (e.g., 1.5 inches), and aninner diameter of approximately 0.9-1.9 inches (e.g., 1.4 inches).

FIG. 9 is a side view of the optical mandrel 1006 of FIGS. 7-8 withoptical fiber 9000 installed, according to an embodiment.

FIG. 10 is a cutaway side view of the optical mandrel 1006 of FIG. 9,according to an embodiment.

Referring to FIGS. 9-10, the first end cap 7004 includes anoptical-fiber path 9002 that extends from the opening 7020 of the firstfiber grip 7000 to an outer perimeter 9004 of the outer receptacle 7044.The optical-fiber path 9002 provides, between the opening 7020 and theouter conduit 7010, a transition that is gradual enough so the bend(s)that the path imparts to an optical fiber within the path do not alter,at least significantly, the propagation or redirection (e.g.,backscattering) properties of the fiber. The optical-fiber path 9002 maybe closed like a conduit, or may be significantly wider than the opticalfiber 9000 in a circumferential direction within the first end cap 7004.

Similarly, the second end cap 7006 includes an optical-fiber path 9012that extends from the opening 7032 of the second fiber grip 7002 to anouter perimeter 9014 of the outer receptacle 7054. The optical-fiberpath 9012 provides, between the opening 7032 and the outer conduit 7010,a transition that is gradual enough so the bend(s) that the path impartsto an optical fiber within the path do not alter, at leastsignificantly, the propagation or redirection (e.g., backscattering)properties of the fiber. The optical-fiber path 9012 may be closed likea conduit, or may be significantly wider than the optical fiber 9000 ina circumferential direction within the second end cap 7006.

And the mandrel 1006 can include a covering, such as shrink wrap, overthe fiber 9000 and outer conduit 7010, and optionally over other partsof the mandrel such as the end caps 7004 and 7006, to protect themandrel and to increase coupling of an acoustic signal incident on themandrel to the optical fiber.

Still referring to FIGS. 9-10, a method for wrapping the fiber 9000about the mandrel is described according to an embodiment in which thefiber 9000 is integral with, that is, forms a single fiber with, theoptical fiber 1010 of the optical-fiber assembly 1004 of FIG. 1, andwhere the optical fiber is conventionally secured to a support member9020, which may be the same as, or similar to, the support member 2006described above in conjunction with FIG. 2. For example purposes, themethod is described where the fiber 9000 is first inserted into theopening of the first fiber grip 7000, it being understood that themethod is similar where the fiber is first inserted into the opening ofthe second fiber grip 7002.

A person, or a machine, inserts the fiber 9000 into the outer opening7022 of the first fiber grip 7000, through the first fiber grip, out ofthe inner opening 7020 of the first fiber grip, into and through theouter end 7046 of the first end cap 7004, into the optical-fiber path9002 of the first end cap, and out of the optical fiber path at theperimeter 9004 of the outer receptacle 7044. The person or machine alsoinserts the support member 9020 through the first fiber grip 7000,through the first end cap 7004, the inner conduit 7008, the second endcap 7006, and the second fiber grip 7002, and out from the outer opening7034 of the second fiber grip.

Next, the person or machine wraps the fiber 9000 around the outersurface of the outer conduit 7010 a number of times that was determinedbeforehand by a designer of the mandrel 1006.

Then, the person or machine inserts the end of the fiber 9000 into theend of the optical-fiber path 9012 at the perimeter 9014 of the outerreceptacle 7054 of the second end cap 7006, through the optical-fiberpath and out the outer end 7056 of the second end cap, into the inneropening 7032 of the second fiber grip 7002, through the second fibergrip, and out from the outer opening 7034 of the second fiber grip.

The person or machine next re-secures the optical fiber 9000 to thesupport member 9020.

In an alternate embodiment, the person or machine may assemble themandrel 1006 after installing the optical fiber 9000 and support member9020 per above.

And where the fiber 9000 is separate, and later spliced to, the opticalfiber 1010 (FIG. 1) of the optical-fiber assembly 1004 (FIG. 1), orwhere the mandrel 1006 is installed at an end of the optical fiber, theperson or machine can wind the fiber 9000 about the outer conduit 7010before inserting either end of the fiber into the respectiveoptical-fiber paths 9002 and 9012.

Still referring to FIGS. 9-10, a method for wrapping the fiber 9000about the mandrel is described according to an embodiment in which thefiber 9000 (mandrel fiber) is separate from the optical fiber 1010(non-mandrel fiber) of the optical-fiber assembly 1004 of FIG. 1, whereeach of the optical fibers 1010 and 9000 is coated with a respectivepolymer and is disposed inside of a respective rubber covering alongwith a respective support member 9020 of fibrous strands as describedabove in conjunction with FIG. 2, and where each of the fiber grips 7000and 7002 is made from multiple (e.g., four) respective pieces.

The fiber 9000 is to be wound about the outer conduit 7010 withapproximately seven hundred fifty turns.

The first fiber grip 7000 is placed over an intact end of one piece ofthe non-mandrel optical fiber 1010 (FIG. 1) to begin assembly of acorresponding first end of the mandrel 1006. Similarly, the second fibergrip 7002 is placed over an intact end of another piece of thenon-mandrel optical fiber 1010 (the mandrel 1006 will be disposed inline with the non-mandrel fiber 1010 between the two pieces) to beingassembly of a corresponding second end of the mandrel.

In this embodiment, each fiber grip 7000 and 7002 is made of fourpieces. A human or machine places a segment 7024 piece of the firstfiber grip 7000 onto the non-mandrel fiber end first, followed by arubber “barrel” piece of the first fiber grip, followed by a piece ofthe first fiber grip with the threads 7026, followed by an O-ring (notshown in FIGS. 7-10) that goes over the threads 7026 such that theO-ring creates a liquid-tight barrier between a hexagonal wall behindthe threads 7026 on which the first fiber grip sits, and the wallagainst which the first fiber grip effectively is forced against whenthe mail threads 7026 are engaged with female threads inside the outeropening 7046 of the first end cap 7004. When assembled, the rubber“barrel” fits into a compression area of the final piece mentioned, andthe flexible, springy piece with the segments 7024 is threaded such thatit goes over the rubber “barrel” inside the compression area. Thetighter the piece with segments 7024 is tightened over the compressionarea, the tighter the rubber “barrel” squeezes on the rubber coating ofthe non-mandrel fiber optic fiber 1010 (FIG. 1). This gives aliquid-tight seal between the rubber coating of the non-mandrel opticalfiber 1010 and the walls of the opening through the middle of the firstfiber grip 7000. These pieces, however, are not tightened at this time,and remain “loose” (e.g., threads not fully engaged until a time asdescribed below.

The person or machine removes the protective rubber coating (e.g., froma section of the non-mandrel fiber about fourteen inches long) from anend of the non-mandrel fiber 1010 that goes in between the mandrel 1006and another adjacent mandrel to expose the support member 9020 (e.g.,fibrous strands). The non-mandrel optical fiber is coated with a polymerthat makes the outside diameter of the fiber-coating combination about900 μm.

Next, the person or the machine pushes the end of the coated non-mandrelfiber 1010 into and through the fiber path 9002 until it exits the fiberpath at perimeter 9004 of the outer receptacle 7044 of the first end cap7004; but the person or machine need not push the entire length (e.g.,fourteen inches) of the exposed section of the non-mandrel fiber throughthe optical-fiber path during this step.

The person or machine trims the support member 9020 of the non-mandrelfiber 1010 to approximately 0.5 inches in length from the end of thepreviously cut rubber fiber cover. That is, the support member 9020 istrimmed such that about one-half inch extends out from the fiber cover.

The person or machine folds the remaining portion (e.g., one-half inch)of the support member 9020 over the threads 7026 of the first fiber grip7000 such that the end of the previously trimmed rubber cover is notquite flush with the end of the threads 7026 (e.g., the previously cutend of the rubber cover is recessed from the inner end 7016 of the firstfiber grip about 0.1 inches)). The rubber covering not being flush withthe end 7016 of the first fiber grip 7000 helps to keep the non-mandreloptical fiber 1010 from becoming twisted in the next step.

After the person or machine folds the end of the support member 9020over the threads 7026 the first fiber grip 7000, the person or machinepushes the non-mandrel fiber 1010 further through the optical-fiber path9002 while the person or machine screws the first fiber grip 7000 intothe outer end 7046 of the first end cap 7004 (thereby capturing theexposed end of the support member 9020 in between the male threads 7026and the female threads of the inner opening 7046 of the first end cap7004). The first fiber grip 7000 is fully engaged with the first end cap7004 when the O-ring (not shown in FIGS. 9-10) that sits on the threads7026 is adequately compressed between the hexagonal wall behind thethreads 7026 and the outer end 7046 of the first end cap 7004. Theexposed end of the support member 9020 being folded over the threads7026, and thus being held between the threads 7026 and the threadsinside the outer end 7046 of the first end cap 7004, keeps thenon-mandrel fiber 1010 from pulling out of the first fiber grip 7000 andpossibly allowing damage to the non-mandrel fiber. While turning thefirst fiber grip 7000 to engage the threads 7026 with the threads insideof the outer end 7046 of the first end cap 7004, the person or machineturn the first fiber grip in a manner such that the portion of thenon-mandrel fiber 1010 inside the optical-fiber path 9002 does not kink.That is, the portion of the non-mandrel fiber 1010 inside theoptical-fiber path 9002 should be turning freely with each turn of thefirst fiber grip 7000 while “screwing” the first fiber grip into thefirst end cap 7004.

After the male threads 7026 are fully engaged with the female threadsinside of the outer end 7046 of the end cap 7004 (i.e., the O-ringbetween the first fiber grip 7000 and the first end cap is sufficientlycompressed), the person or machine screws the section of the first fibergrip with the segments 7024 onto the compression area (over the rubber“barrel” and end piece of the first fiber grip) until the rubber“barrel” is snuggly squeezing the rubber coating of the non-mandreloptic fiber 1010.

The person or machine may pull the rubber cover of the non-mandrel opticfiber 1010 (FIG. 1) slightly in a direction away from the first end cap7004 while the rubber “barrel” is being compressed onto the rubbercover. This biases the non-mandrel fiber such that if it is subsequentlypulled or strained during deployment, the fiber 1010 is already stressedagainst the support member 9020 (being held by the first-fiber-gripthreads 7026), and won't experience a sudden “jerk” (for example,imagine railroad cars spread apart (tensioned) such that when the enginetakes off, it doesn't cause a series of jerks and jolts down the line ofcars).

The person or machine can now strip the coating off of the portion ofthe non-mandrel optical fiber 1010 extending from the path 9002 at theperimeter 9004 of the inner receptacle 7044 of the first end cap 7004,and splice the end of the non-mandrel fiber to a corresponding end ofthe mandrel fiber 9000, which the person or machine has already woundabout the outer conduit 7010 per above. The person or machine may firstneed to unwind about five turns of the mandrel winding 9000 at thecorresponding end of the outer conduit 7010 so that there is “room” tofit the ends of the mandrel and non-mandrel fibers 9000 and 1010 intothe fusion splicing machine, which splices together the ends of themandrel and non-mandrel fibers.

After splicing the non-mandrel fiber 1010 with the mandrel fiber 9000,there is a significant amount of fiber that is now “hanging free” (i.e.,it isn't wrapped around the outer conduit 7010). As the person ormachine inserts the corresponding end of the outer conduit 7010 into theouter receptacle 7044 of the inner end 7040 of the first end cap 7004,he/she/it spins the outer conduit along its longitudinal axis “to takeup all the slack” in the spliced optical fiber, thereby wrapping all ofthe fiber slack onto the outer conduit.

Next, the person or machine screws the inner conduit 7008 into the innerreceptacle 7052 of the inner end 7050 of the second end cap 7006.

The person or machine repeats the above steps (but for screwing theinner conduit 7008 into the inner receptacle 7052) for the second fibergrip 7002, the second end cap 7006, the other end of the mandrel opticalfiber 9000, and the end of another section of the non-mandrel opticalfiber 1010 (FIG. 1) to assemble the second end of the mandrel 1006 andto splice the other end of the mandrel optical fiber to the othersection of the non-mandrel optical fiber.

As the person or machine inserts the corresponding end of the outerconduit 7010 into the outer receptacle 7054 of the inner end 7050 of thesecond end cap 7006, and spins the outer conduit along its longitudinalaxis “to take up all the slack” in the spliced optical fiber, the personor machine is also simultaneously screwing the other end of the innerconduit 7008 into the inner receptacle 7042 of the inner end 7040 of thefirst end cap 7004. A snug fit is achieved in all fittings, with theinner conduit 7008 securely screwed into both end caps 7004 and 7006,and the O-rings 7012 and 7014 well seated between the ends of the outerconduit 7010 and a channel or slot inside the outer receptacles 7040 and7050 at the respective perimeters 9004 and 9014.

The person or machine rotates first and second end caps 7004 and 7006such that the spliced fiber ends do not experience excessive strain;that is, the person or machine winds the fiber tight enough so thatthere are no overlapping fiber turns, but just loose enough so that theturns of the fiber can “glide” slightly over the outer conduit 7010.

Next, the person or machine applies a single, protective layer ofstandard electrical tape (not shown in FIGS. 7-10), with a minimum ofoverlap between each turn of the tape, over the entire outer conduit7010. The purpose of this protective layer is to protect, during thenext assembly step, the fragile glass optical fiber wound around theouter conduit 7010.

The person or machine then covers the mandrel 1006 with a no-heat shrinkwrap (not shown in FIGS. 7-10), which is made from an expanded rubbertube that has been expanded and that is supported by a rigid, spiral,plastic member that is disposed inside of, and that extends the lengthof, the rubber tube. Shrinking of the wrap occurs when the person ormachine pulls (e.g., rip-cord style) the rigid, spiral plastic memberfrom the inside of the rubber tube. As the person or machine removes theinternal plastic member, the shrink wrap starts to shrink around theouter conduit 7010 (and possibly other sections such as the end caps7004 and 7006 and the fiber grips 7000 and 7002) of the mandrel 1006.The electrical tape covering the optical fiber wound around the outerconduit 7010 protects the fiber from the rigid plastic member passingover the fiber, sometimes rather vigorously.

The end result is a fluid-tight, or a liquid-tight, mandrel.

This above-described process for assembling a mandrel 1006 is repeatedfor every other mandrel in the optical-fiber assembly 1004 (FIG. 1).

Referring to FIGS. 7-10, alternate embodiments of the mandrel 1006 arecontemplated. For example, the components of the mandrel 1006 can haveany suitable dimensions other than those described above. Furthermore,the mandrel 1006 can be used with an optical fiber 9000 that is notsecured to a support member 9020. Moreover, embodiments described inconjunction with FIGS. 1-6 and 11-25 may be applicable to the mandrel1006 of FIGS. 7-10.

FIG. 11 is an isometric view of an optical mandrel 1006 of FIG. 1,according to another embodiment.

FIG. 12 is an exploded view of the optical mandrel 1006 of FIG. 11,according to an embodiment.

Referring to FIGS. 11-12, as described below, like the optical mandrel1006 of FIGS. 7-10, the optical mandrel 1006 of FIGS. 11-12 isconfigured to be disposed “in-line” with the optical fiber 1010 in theoptical-fiber assembly 1004 of FIG. 1, and includes optical-fiber paths(not visible in FIGS. 11-12) that are configured to bend the opticalfiber gradually enough to prevent the fiber bends from changing thepropagating and redirecting (e.g., backscattering) characteristics ofthe optical fiber as compared the redirecting characteristics of astraight span of the fiber. Furthermore, the optical mandrel 1006 isdesigned to amplify acoustic signals incident on the mandrel such thatthe acoustic signals that the electro-optic sensor subsystem 1002 ofFIG. 1 recovers from the redirected optical signal are more likely tohave a higher optical power and a higher SNR as compared to acousticsignals that the sensor subsystem 1002 recovers from non-mandrel zonesof the optical fiber 1010 of FIG. 1.

The optical mandrel 1006 includes first and second fiber grips 11000 and11002, first and second end caps 11004 and 11006, an inner conduit11008, and outer conduit 11010, first and second O-rings 11012 and11014, and third and fourth O-rings 11015 and 11017.

The first fiber grip 11000 has inner and outer ends 11016 and 11018 withrespective inner and outer openings 11020 (not visible in FIGS. 11-12)and 11022 configured to receive an optical fiber (not shown in FIGS.11-12), is made from a flexible material such as a rubber and isotherwise configured to be flexible, and is configured to prevent theoptical fiber from breaking, or from otherwise becoming damaged, as thefiber transitions between a non-mandrel span and the mandrel 1006; forexample, the first fiber grip 11000 can include spaced segments 11024,which increase the flexibility of the first fiber grip as compared to asegment-less fiber grip. Furthermore, the inner end 11016 is configuredfor attachment to the first end cap 11004; for example, the inner endcan include male threads 11026 for engaging female threads (not visiblein FIGS. 11-12) of the first end cap. Moreover, the inner and outeropenings 11020 and 11022 are configured to make a fluid-tight, orliquid-tight, seal with the optical fiber that extends through the firstand second openings. In addition, the first fiber grip 11000 can have alength of approximately 1.5-2.5 inches (e.g., 2.0 inches), an innerdiameter of approximately 0.1-0.2 (e.g., 0.16 inches), and an outerdiameter of approximately 0.6-1.0 inches (e.g., 0.8 inches).

Similarly, the second fiber grip 11002 has inner and outer ends 11028and 11030 with respective inner and outer openings 11032 (not visible inFIGS. 11-12) and 11034 configured to receive an optical fiber (not shownin FIGS. 11-12), is made from a flexible material such as a rubber andis otherwise configured to be flexible, and is configured to prevent theoptical fiber from breaking, or from otherwise becoming damaged, as thefiber transitions between a non-mandrel span and the mandrel 1006; forexample, the second fiber grip 11002 can include spaced segments 11036,which increase the flexibility of the first fiber grip as compared to asegment-less fiber grip. Furthermore, the inner end 11028 is configuredfor attachment to the second end cap 11006; for example, the inner endcan include male threads 11038 for engaging female threads (not visiblein FIGS. 11-12) of the first end cap. Moreover, the inner and outeropenings 11032 and 11034 are configured to make a fluid-tight, orliquid-tight, seal with the optical fiber that extends through the firstand second openings. In addition, the second fiber grip 11002 can have alength of approximately 1.5-2.5 inches (e.g., 2.0 inches), an innerdiameter of approximately 0.1-0.2 (e.g., 0.16 inches), and an outerdiameter of approximately 0.6-1.0 inches (e.g., 0.8 inches).

The first end cap 11004 includes an inner end 11040, which includes aninner receptacle 11042 and an outer receptacle 11044, and includes anouter end 11046, which is a separate piece from the inner end. The firstend cap 11004 can be formed from any suitable material such as metal,plastic, or resin. The inner receptacle 11042 is configured forattachment to an end of the inner conduit 11008; for example, the innerreceptacle may include female threads configured to engage male threadson the end of the inner conduit. The outer receptacle 11044 isconfigured for receiving an end of the outer conduit 11010; for example,the outer receptacle can be flanged so as to fit snugly around the endof the outer conduit, which, when installed, presses against a back ofthe outer receptacle to provide a fluid-tight or liquid-tight seal thatprevents contaminants such as dirt and water from entering the innercavity formed by the outer conduit. The outer end 11046 is configuredfor receiving and attaching to the inner end 11016 of the first fibergrip 11000; for example, as described above, the outer end may includefemale threads configured for engaging male threads 11026 of the firstfiber grip. The inner end 11040 is configured to fit into, and attachto, the outer end 11046; for example, the inner and outer ends mayinclude male and female threads, respectively, such that the inner endscrews into the outer end. The O-rings 11012 and 11015 are configured toform seals between the abutting surfaces of the connected inner andouter ends 11040 and 11046 of the first end cap 11004. In addition, thefirst end cap 11004 can have a length of approximately 2.0-3.0 inches(e.g., 2.5 inches), the inner receptacle 11042 can have a diameter ofapproximately 0.4-0.8 inches (e.g., 0.6 inches), the outer receptacle11044 can have a diameter of approximately 1.3-1.7 inches (e.g., 1.5inches), and the outer end 11046 can have an inner diameterapproximately equal to the diameter of the inner receptacle and can havean outer diameter of approximately 1.0-2.0 inches (e.g., 1.25 inches).

Similarly, the second end cap 11006 includes an inner end 11050, whichincludes an inner receptacle 11052 and an outer receptacle 7054, andincludes an outer end 11056. The second end cap 11006 can be formed fromany suitable material such as metal, plastic, or resin. The innerreceptacle 11052 is configured for attachment to an end of the innerconduit 11008; for example, the inner receptacle may include femalethreads configured to engage male threads on the end of the innerconduit. The outer receptacle 11054 is configured for receiving an endof the outer conduit 11010; for example, the outer receptacle can beflanged so as to fit snugly around the end of the outer conduit, which,when installed, presses against the back of the outer receptacle toprovide a fluid-tight or liquid-tight seal that prevents contaminantssuch as dirt and water from entering the inner cavity formed by theouter conduit. The outer end 11056 is configured for receiving andattaching to the inner end 11028 of the second fiber grip 11002; forexample, as described above, the outer end may include female threadsconfigured for engaging male threads 11038 of the second fiber grip. Theinner end 11050 is configured to fit into, and attach to, the outer end11056; for example, the inner and outer ends may include male and femalethreads, respectively, such that the inner end screws into the outerend. The O-rings 11014 and 11017 are configured to form seals betweenthe abutting surfaces of the connected inner and outer ends 11050 and11056 of the second end cap 11006. In addition, the second end cap 11006can have a length of approximately 2.0-3.0 inches (e.g., 2.5 inches),the inner receptacle 11052 can have a diameter of approximately 0.4-0.8inches (e.g., 0.6 inches), the outer receptacle 11054 can have adiameter of approximately 1.3-1.7 inches (e.g., 1.5 inches), and theouter end 11056 can have an inner diameter approximately equal to thediameter of the inner receptacle and can have an outer diameter ofapproximately 1.0-2.0 inches (e.g., 1.25 inches).

The inner conduit 11008 is a hollow pipe nipple that includes first andsecond ends 11060 and 11062. The inner conduit 11008 can be made fromany suitable material, such as metal or plastic. Each of the first andsecond ends 11060 and 11062 are configured for attachment to the innerreceptacles 11042 and 11052 of the first and second end caps 11004 and11006, respectively; for example, as described above, the first andsecond ends of the inner conduit 11008 can have male threads 11064 and11066 configured to engage female threads of the inner receptacles 11042and 11052, respectively. It is the inner conduit 11008 that couplestogether the first and second end caps 11004 and 11006, and, therefore,that “holds the mandrel 1006 together.” The first and second ends 11060and 11062 can be configured to make a fluid-tight or a liquid-tight sealwith the inner receptacles 11042 and 11052 of the first and second endcaps 11004 and 11006, respectively. Furthermore, the inner conduit 11008can have a length of approximately 8.0-10.0 inches (e.g., 8.8 inches),an outer diameter of approximately 0.6-0.8 inches (e.g., 0.7 inches),and an inner diameter of approximately 0.5-0.7 inches (e.g., 0.6inches).

The outer conduit 11010 is a cylinder or tube that includes first andsecond ends 11070 and 11072 and is the component of the mandrel 1006about which an optical fiber (e.g., the optical fiber 1010 of FIG. 1) iswound. The outer conduit 11010 can be made from any suitable material,such as metal or plastic. And although shown as being transparent tolight, the outer conduit 11010 may be opaque. Each of the first andsecond ends 11070 and 11072 are configured for fitting into the outerreceptacles 11044 and 11054 of the first and second end caps 11004 and11006 and pressing against the backs of the outer receptacles,respectively. The first and second ends 11070 and 11072, therefore, makerespective fluid-tight or liquid-tight seals with the backs of the outerreceptacles 11044 and 11054, respectively, to prevent contaminants suchas water and dirt from entering the cavity formed by the outer conduit11010. Alternatively, O-rings, or other sealing members (not shown inFIGS. 11-12), may be disposed between the ends 11070 and 11072 and thebacks of the outer receptacles 11044 and 11054 to assist in formingrespective fluid-tight or liquid-tight seals. Furthermore, the outerconduit 11010 can have a length of approximately 8.0-10.0 inches (e.g.,8.8 inches), an outer diameter of approximately 1.0-2.0 inches (e.g.,1.5 inches), and an inner diameter of approximately 0.9-1.9 inches(e.g., 1.4 inches).

And the O-rings 11012, 11014, 11015, and 11017 can be conventional andmade from any suitable material such as a rubber. Furthermore, each ofthe O-rings 11012, 11014, 11015, and 11017 can have inner and outerdiameters that are approximately the same as the inner and outerdiameter of the outer conduit 11010, that is, an outer of approximately1.0-2.0 inches (e.g., 1.5 inches), and an inner diameter ofapproximately 0.9-1.9 inches (e.g., 1.4 inches).

FIG. 13 is a cutaway side view of the optical mandrel 11006 of FIGS.11-12 with optical fiber 13000 installed, according to an embodiment.

FIG. 14 is an isometric view of the optical mandrel 1006 of FIGS. 11-13with portions transparent, according to an embodiment.

Referring to FIGS. 13-14, the first end cap 11004 includes anoptical-fiber path 13002 that extends from the opening 11020 (FIGS.11-12) of the first fiber grip 11000 to an outer perimeter 13004 of theouter receptacle 11044. The optical-fiber path 13002 provides, betweenthe opening 11020 and the outer conduit 11010, a transition that isgradual enough so the bend(s) that the path imparts to an optical fiberwithin the path do not alter, at least significantly, the propagation orredirection (e.g., backscattering) properties of the fiber. Theoptical-fiber path 13002 may be closed like a conduit, or may besignificantly wider than the optical fiber 13000 in a circumferentialdirection within the first end cap 11004.

Similarly, the second end cap 11006 includes an optical-fiber path (notvisible in FIGS. 13-14) that extends from the opening 11032 of thesecond fiber grip 11002 to an outer perimeter 13014 of the outerreceptacle 11054. The optical-fiber path of the second end cap 11006provides, between the opening 11032 and the outer conduit 11010, atransition that is gradual enough so the bend(s) that the path impartsto an optical fiber within the path do not alter, at leastsignificantly, the propagation or redirection (e.g., backscattering)properties of the fiber. The optical-fiber path may be closed like aconduit, or may be significantly wider than the optical fiber 13000 in acircumferential direction within the second end cap 11006.

And the mandrel 1006 can include a covering (not shown in FIGS. 13-14),such as shrink wrap, over the fiber 13000 and outer conduit 11010, andoptionally over other parts of the mandrel such as the end caps 11004and 11006, to protect the mandrel and to increase coupling of anacoustic signal incident on the mandrel to the optical fiber.

Still referring to FIGS. 13-14, a method for wrapping the fiber 13000about the mandrel 1006 is described according to an embodiment in whichthe fiber 13000 is integral with, that is, forms a single fiber with,the optical fiber 1010 of the optical-fiber assembly 1004 of FIG. 1, andwhere the optical fiber is conventionally secured to a support member13020. For example purposes, the method is describe where the fiber13000 is first inserted into the opening of the first fiber grip 11000,it being understood that the method is similar where the fiber is firstinserted into the opening of the second fiber grip 11002.

A person, or a machine, inserts the fiber 13000 into the outer opening11022 of the first fiber grip 11000, through the first fiber grip, outof the inner opening 11020 of the first fiber grip, into and through theouter end 11046 of the first end cap 11004, into the optical-fiber path13002 of the first end cap, and out of the optical fiber path at theperimeter 13004 of the outer receptacle 11044. The person or machinealso inserts the support member 13020 through the first fiber grip11000, through the first end cap 11004, the inner conduit 11008, thesecond end cap 11006, and the second fiber grip 11002, and out from theouter opening 11034 of the second fiber grip.

Next, the person or machine wraps the fiber 13000 around the outersurface of the outer conduit 11010 a number of times that was determinedbeforehand by a designer of the mandrel 1006.

Then, the person or machine inserts the end of the fiber 13000 into theend of the optical-fiber path (not visible in FIGS. 13-14) of the secondend cap 11006 at the perimeter 13014 of the outer receptacle 11054 ofthe second end cap, through the optical-fiber path and out the outer end11056 of the second end cap, into the inner opening 11032 of the secondfiber grip 11002, through the second fiber grip, and out from the outeropening 11034 of the second fiber grip.

The person or machine next re-secures the optical fiber 13000 to thesupport member 13020 adjacent to the second fiber grip 11002.

In an alternate embodiment, the person or machine may assembly themandrel 1006 after installing the optical fiber 13000 and support member13020 per above.

And where the fiber 13000 is separate, and later spliced to, the opticalfiber 1010 (FIG. 1) of the optical-fiber assembly 1004 (FIG. 1), orwhere the mandrel 1006 is installed at an end of the optical fiber, theperson or machine can wind the fiber 13000 about the outer conduit 11010before inserting either end of the fiber into the respectiveoptical-fiber path 13002 of the first end cap 11004 and the opticalfiber path (not visible in FIGS. 13-14) of the second end cap 11006.

Referring to FIGS. 11-14, alternate embodiments of the mandrel 1006 arecontemplated. For example, the components of the mandrel 1006 can haveany suitable dimensions other than those described above. Furthermore,the mandrel 1006 can be used with an optical fiber 13000 that is notsecured to a support member 13020. Moreover, embodiments described inconjunction with FIGS. 1-10 and 15-25 may be applicable to the mandrel1006 of FIGS. 11-14.

Description of Underlying Theories Heterodyne Optical Detection

Optical receivers are built around photodetectors which detect opticalpower rather than instantaneous electric field. Typically, thephotodetector output current is proportional to the incident opticalpower. This relationship severely limits the dynamic range of anincoherent optical receiver because for every decibel of optical powerlost in a receiver system two decibels of receiver output current islost. The square law characteristics of photodetectors limits typicalincoherent optical receivers (often called video detection receivers) todynamic ranges of less than 80 dB and optical detection noise floors togreater than −80 dBm per Hertz bandwidth. As illustration, suppose anelectric field E_(s)(t) [volt/meter] immersed in a material of impedanceη[Ohms] impinges upon a photodetector of responsivity R [ampere/watt]loaded by resistor Ri and amplified by amplification A, then the opticalpower P_(s) by amplification A, is:

$\begin{matrix}{{P_{S}(t)} = \frac{\left( {{\overset{\rightarrow}{E_{S}}(t)} \cdot {{\overset{\rightarrow}{E}}_{S}(t)}} \right)}{\eta}} & (1)\end{matrix}$

The photodetector output current [amperes] is:

i(t)=

P _(S)(t)  (2)

The photoreceiver output [volts] is thus:

v(t)=AR _(L) i(t)=AR _(L)

P _(S(t))  (3)

The output fades only if the optical signal power goes to zero becausethe vector dot product of an optical signal against itself has nopolarization or phase effects. This lack of fading due to polarizationor phase comes at a cost: phase information is lost and signal to noiseratios are severely impacted.

A coherent optical receiver takes advantage of the square lawcharacteristics of photodetectors. A coherent optical receiver combinestwo optical beams, a signal and a local oscillator, together to form aninterference. The interference between these optical waves produces a“beat” which allows the measurement of the phase difference between thesignal and the local oscillator. This interference produces anamplitude, polarization, and phase sensitive receiver output. In orderto consider these effects a discussion of the polarization state ofplane waves is in order. A plane wave contains two orthogonal vectorcomponents which are also orthogonal to the direction of propagation ofthe wave. For purposes of discussion we will consider the plane wave tobe oriented so that the vector components of the electromagnetic fieldlie in an X-Y plane and that the wave propagates in the Z direction.However, this choice of axes is completely arbitrary. In practice, thewave can be oriented in any propagation direction. In order to simplifydiscussions, a simple change of coordinates will make this discussioncompletely general.

The polarization of an electromagnetic (or optical) plane wave, p, isdescribed by a minimum of five parameters. There are two basic ways ofspecifying these parameters. The first way leads to a description whichis oriented towards that which is directly obtained from physicalmeasurements.

$\begin{matrix}{{{\overset{\rightarrow}{E}}_{p}\left( {E_{p\; x},E_{p\; y},\Phi_{p\; x},\Phi_{p\; y},\omega_{p},t} \right)} = \begin{bmatrix}{{E_{p\; x}(t)}{\cos\left( {{\omega_{p}t} + \Phi_{p\; x}} \right)}} \\{{E_{p\; y}(t)}{\cos\left( {{\omega_{p}t} + \Phi_{p\; y}} \right)}}\end{bmatrix}} & (4)\end{matrix}$

The second manner of describing the polarization state of a wave, p, isoriented more towards the underlying physical mechanisms ofpolarization. See FIG. 15. The description is made in terms of spatialand temporal parameters:

$\begin{matrix}{{{\overset{\rightarrow}{E}}_{p}\left( {E_{p},\theta_{p},\psi_{p},\phi_{p},\omega_{p},t} \right)} = {{{{E_{p}(t)}\begin{bmatrix}{\cos\left( \theta_{p} \right)} & {\sin\left( \theta_{p} \right)} \\{- {\sin\left( \theta_{p} \right)}} & {co{s\left( \theta_{p} \right)}}\end{bmatrix}}\begin{bmatrix}{\cos\left( \psi_{p} \right)} & 0 \\0 & {\sin\left( \psi_{p} \right)}\end{bmatrix}}\begin{bmatrix}{\cos\left( {{\omega_{p}t} + \phi_{p}} \right)} \\{\sin\left( {{\omega_{p}t} + \phi_{p}} \right)}\end{bmatrix}}} & (5)\end{matrix}$

Alternatively, dropping the full variable list in the parentheses andexpanding:

$\begin{matrix}{{{\overset{\rightarrow}{E}}_{p}(t)} = \begin{matrix}{{E_{p}(t)}\begin{bmatrix}{co{s\left( \theta_{p} \right)}} & {si{n\left( \theta_{p} \right)}} \\{{- s}i{n\left( \theta_{p} \right)}} & {co{s\left( \theta_{p} \right)}}\end{bmatrix}} & \begin{bmatrix}{co{s\left( \psi_{p} \right)}} & 0 \\0 & {si{n\left( \psi_{p} \right)}}\end{bmatrix} \\\begin{bmatrix}{co{s\left( \theta_{p} \right)}} & {{- s}i{n\left( \theta_{p} \right)}} \\{si{n\left( \theta_{p} \right)}} & {co{s\left( \theta_{p} \right)}}\end{bmatrix} & \begin{bmatrix}{co{s\left( {\omega_{p}t} \right)}} \\{si{n\left( {\omega_{p}t} \right)}}\end{bmatrix}\end{matrix}} & (6)\end{matrix}$

If Ep is constant, the electrical power of this wave can be shown to beconstant and equal to:

$\begin{matrix}{{P_{p}(t)} = {\frac{\left( {{{\overset{\rightarrow}{E}}_{p}(t)} \cdot {{\overset{\rightarrow}{E}}_{p}(t)}} \right)}{\eta} = \frac{E_{p}^{2}}{2\eta}}} & (7)\end{matrix}$

When two waves, S (signal) and L (local oscillator), interfere at theinput of a photoreceiver, the output is:

$\begin{matrix}{{{V_{out}(t)} = {{AR_{L}{i(t)}} = {AR_{L}\Re\frac{\left\langle {{{{\overset{\rightarrow}{E}}_{S}(t)} \cdot {{\overset{\rightarrow}{E}}_{S}(t)}} + {{{\overset{\rightarrow}{E}}_{L}(t)} \cdot {{\overset{\rightarrow}{E}}_{L}(t)}} + {2{{{\overset{\rightarrow}{E}}_{L}(t)} \cdot {{\overset{\rightarrow}{E}}_{S}(t)}}}} \right\rangle}{\eta}}}}\mspace{79mu}{{V_{out}(t)} = {{{V_{L}(t)} + {V_{S}(t)} + {V_{LS}(t)}} = {AR_{L}{\Re\left( {{P_{L}(t)} + {P_{s}(t)} + {P_{LS}(t)}} \right)}}}}} & (8)\end{matrix}$

If the optical power of the local oscillator and signal light wavesremain constant, a constant photocurrent develops for theself-interference terms (P_(S) and P_(L)). However, if either the localoscillator or the signal light waves have any temporal variation inpolarization or phase, the cross-interference term (P_(LS)) will be timedependent even if the power of each light wave remains constant. Solvingfor the cross-interference term, we obtain:

$\begin{matrix}{{{v_{LS}(t)} = {\frac{AR_{L}\Re}{\eta}{E_{L}(t)}{{E_{s}(t)}\begin{bmatrix}{{\cos({\Delta\theta})}{\cos({\Delta\psi})}{\cos\left( {{{\Delta\omega}t} + {\Delta\phi}} \right)}} \\{{+ {\sin({\Delta\theta})}}{\sin\left( {2\overset{¯}{\psi}} \right)}{\sin\left( {{{\Delta\omega}t} + {\Delta\phi}} \right)}}\end{bmatrix}}}}{{v_{LS}(t)} = {2AR_{L}\Re{\sqrt{{P_{L}(t)}{P_{s}(t)}}\begin{bmatrix}{{{\cos({\Delta\theta})}{\cos({\Delta\psi})}{\cos\left( {{\Delta\omega t} + {\Delta\phi}} \right)}} +} \\{{\sin({\Delta\theta})}{\sin\left( {2\overset{¯}{\psi}} \right)}{\sin\left( {{\Delta\omega t} + {\Delta\phi}} \right)}}\end{bmatrix}}}}} & (9)\end{matrix}$

Where the following definitions are made:

Δθ=θ_(S)−θ_(L)

Δ_(ψ)=ψ_(S)−ψ_(L)

2ψ=ψ_(S)+ψ_(L)

Δψ=ω_(S)−ω_(L)

Δϕ=ϕ_(S)−ϕ_(L)  (10)

The optical cross-interference portion of the receiver output will fadedue to polarization even if the local oscillator and the signal lightwaves both do not have zero optical powers. This condition will occurif:

0=cos(Δθ)cos(Δψ)cos(Δωτ+Δϕ)=sin(Δθ)sin(2ψ)sin(Δωt+ΔΦ)  (11)

Also, equivalently when the condition will occur:

$\begin{matrix}{\begin{bmatrix}0 \\0\end{bmatrix} = \begin{bmatrix}{{\cos({\Delta\theta})}{\cos({\Delta\psi})}{\cos\left( {{\Delta\omega t} + {\Delta\phi}} \right)}} \\{{\sin({\Delta\theta})}{\sin\left( {2\psi} \right)}{\sin\left( {{{\Delta\omega}t} + {\Delta\phi}} \right)}}\end{bmatrix}} & (12)\end{matrix}$

When heterodyne optical detection is employed (Δω is non-zero, the localoscillator has a different frequency from the signal), the conditionsfor a fade are shown in Table 1. When homodyne detection is employed (Δωis zero), both phase and polarization fading occur. The conditions for ahomodyne fade are shown in Table 2. Heterodyne detection is, therefore,seen to be superior to homodyne because the probability of a fade isfully one half as likely.

TABLE 1 Heterodyne Fading Conditions Type of Face Required Simultaneous(k is an integer) Conditions for a Fade to Occur Orthogonal Rotation andΔσ = (2k + 1)π/2 Ψ_(s) + Ψ_(L) = 0 Opposite Ellipticity OrthogonalRotation and Δσ = (2k + 1)π/2 Ψ_(s) + Ψ_(L) ± π Equal CircularEllipticity Equal Rotation and Δσ = 0 ΔΨ = ± π/2 Orthogonal EllipticityOpposite Rotation and Δσ = ±π ΔΨ = ± π/2 Orthogonal Ellipticity

TABLE 2 Homodyne Fading Conditions Type of Face Required Simultaneous (kand m are integers) Conditions for a Fade to Occur Orthogonal Rotationand Δσ = (2k + 1)π/2 Ψ_(s) + Ψ_(L) = 0 Opposite Ellipticity OrthogonalRotation and Δσ = (2k + 1)π/2 Ψ_(s) + Ψ_(L) = 0 Equal CircularEllipticity Equal or Opposition Rotation Δσ = kπ ΔΨ = ± π/2 andOrthogonal Ellipticity Opposite Rotational and Δσ = (2k + 1)π/2 ΔΦ = mπEqual or Opposite Phase

Given the conditions for and the functional relation of a fade, thequestion now arises as to how a fade can be prevented. Since the signalis being measured, no a prior knowledge is assumed and therefore E_(s),θ_(s), Ψ_(s), Φ_(s) are all probably unknown quantities. If fading isprevented, then no loss of information occurs and determination of thesefour parameters is possible. In order to decode the optical receiveroutput into these parameters, at least four independent measurementsmust be made to uniquely determine these four independent variables.However, if the interfering optical beam (or beams) of the localoscillator are unknown, then additional independent measurements aremade (four additional measurements for each unknown beam) to determinethe E_(L), θ_(L), Ψ_(L), Φ_(L) for each optical beam of the localoscillator. The cross-reference output of the photoreceiver, V_(LS)(t),offers a means by which to measure these parameters. If the parameterscannot be determined from this output, then an optical fade cannot beruled out.

We shall now examine the information which can be gleaned from thisoutput. Define the following functions.

$\begin{matrix}{{{v_{I}\left( {E_{L},E_{S},{\Delta\theta},\Delta_{\psi}} \right)} = {{\frac{AR_{L}\Re}{2_{\eta}}{E_{L}(t)}{E_{S}(t)}{\cos\left( {\Delta\theta} \right)}{\cos\left( {\Delta\psi} \right)}} = {AR_{L}\Re\sqrt{{P_{L}(t)}{P_{S}(t)}}{\cos\left( {\Delta\theta} \right)}{\cos\left( {\Delta\psi} \right)}}}}{{v_{Q}\left( {E_{L},E_{S},{\Delta\theta},2_{\overset{¯}{\psi}}} \right)} = {{\frac{AR_{L}R}{2_{\eta}}{E_{L}(t)}{E_{S}(t)}{\sin\left( {\Delta\theta} \right)}{\sin\left( {2\overset{¯}{\psi}} \right)}} = {AR_{L}\Re\sqrt{{P_{L}(t)}{P_{S}(t)}}{\sin\left( {\Delta\theta} \right)}{\sin\left( {2\overset{¯}{\psi}} \right)}}}}} & (13)\end{matrix}$

In the homodyne case (Δω is zero), we obtain the following output:

v _(LS)(t)=2AR _(L)

√{square root over (P _(L)(t)P_(S)(t))}(cos(Δθ)cos(Δϕ)cos(Δθ)+sin(Δθ)sin(2ψ)sin(Δϕ))

v _(LS)(t)=2v _(I)(E _(L) ,E _(S),Δθ,Δψ)cos(Δϕ)+2v _(Q)(E _(L) ,E_(S),Δθ,2ψ)sin(Δθ)  (14)

The homodyne output only allows the measurement of one quantity. Theoutput provides only one independent measurement (one equation) whereasa minimum of four are typically required. In the heterodyne case (Δω isnon-zero), the output is:

$\begin{matrix}{{{v_{LS}(t)} = {2AR_{L}\Re\sqrt{{P_{L}(t)}{P_{s}(t)}}\begin{pmatrix}{{co{s\left( {\Delta\theta} \right)}{\cos({\Delta\psi})}{\cos\left( {{{\Delta\omega}t} + {\Delta\phi}} \right)}} +} \\{{\sin({\Delta\theta})}{\sin\left( {2\overset{¯}{\psi}} \right)}{\sin\left( {{{\Delta\omega}t} + {\Delta\phi}} \right)}}\end{pmatrix}}}\mspace{79mu}{{v_{LS}(t)} = {\frac{AR_{L}\Re}{2\eta}{E_{L}(t)}{E_{s}(t)}\begin{pmatrix}{{{\cos({\Delta\theta})}{\cos({\Delta\psi})}{\cos\left( {{{\Delta\omega}t} + {\Delta\phi}} \right)}} +} \\{{\sin({\Delta\theta})}{\sin\left( {2\overset{¯}{\psi}} \right)}{\sin\left( {{{\Delta\omega}t} + {\Delta\phi}} \right)}}\end{pmatrix}}}{{v_{LS}(t)} = {{2{v_{I}\left( {E_{L},E_{S},{\Delta\theta},{\Delta\psi}} \right)}{\cos\left( {{\Delta\omega t} + {\Delta\phi}} \right)}} + {2{v_{Q}\left( {E_{L},E_{S},{\Delta\theta},\ {2\overset{¯}{\psi}}} \right)}{\sin\left( {{\Delta\omega t} + {\Delta\phi}} \right)}}}}} & (15)\end{matrix}$

Since sine and cosine waves are orthogonal, the heterodyne receiverprovides two independent measurements by mixing down to baseband the Δωradian frequency components. Thus, two outputs are obtained:

V ₁(t)≤v _(LS)(t)cos(Δωt)≥v _(I)(E _(L)(t),E_(S)(t),Δθ(t),Δψ(t))cos(Δϕ(t))

V _(Q)(t)≤v _(LS)(t)sin(Δωt)≥v _(Q)(E _(L)(t),E_(S)(t),Δθ(t),2ψ(t))sin(Δϕ(t))

Correlation or Time-Delay Multiplexing

In many optical sensor applications, the light-wave signalheterodyne-detected by the photodetector system is a composite opticalsignal formed from the superposition of many individual optical signals.When the receiver light wave (redirected optical signal) is generated bybackscatter (of the source optical signal), the redirected opticalsignal is the composite, or superposition, of individual light signalsgenerated by a continuum of reflections (redirections) of aninterrogation (source) light signal. The temporal and spatialcharacteristics of each reflector or reflective region in the opticalfiber creates a modulation of the source optical signal. The time-delay,amplitude, polarization and phase states control thebackscattered-modulation of these individual optical signals arriving atthe photodetector with a unique time-delay interval such that theseindividual optical signals can be separated into channels that sortthese redirected optical signals, or components of the redirectedoptical signal, into time-delay slots or bins. Depending upon how thesignals are generated, these channels can represent spatial regions inspace or time-delay slots of a time-domain reflectometer mechanism suchas an optical fiber.

Let an interrogation light wave source be generated by modulating theamplitude, phase or polarization of a coherent light wave with atime-structured correlation code, c(t). The correlation code, c(t) canbe a series of pulses, chirps, binary sequences or any other type ofcode which provides the required correlation characteristics. If thelight-wave source is:

E _(SS)(t)=E _(SS) cos(ω_(s) t)  (17)

Then an amplitude modulated interrogation source is:

E _(i)(t)=μ_(A) c(t)E _(ss) cos(ω_(s) t)  (18)

Alternatively, a phase modulated interrogation source is:

E _(i)(t)=E _(SS) cos(ω_(S) t+μ _(p) c(t))  (19)

If c(t) is chosen to be temporally structured properly, then:

$\begin{matrix}{{R_{i}(\tau)} = {\left\langle {{E_{i}(t)}{E_{i}\left( {t + \tau} \right)}} \right\rangle \approx \left\{ \begin{matrix}{\frac{E_{SS}^{2}}{2};{\tau \approx 0}} \\{0;}\end{matrix} \right.}} & (20)\end{matrix}$

Otherwise, c(t) is typically chosen so that an aprioridecoding/demultiplexing function, d(t), exists such that:

$\begin{matrix}{{b\left( {t,\tau} \right)} = {\left\langle {{d(t)}{E_{i}\left( {T + \tau} \right)}} \right\rangle \approx \left\{ {\begin{matrix}{{\xi\; E_{SS}{\cos\left( {{{\Delta\omega}\; t} + \phi} \right)}};{\tau \approx 0}} \\0\end{matrix};{otherwise}} \right.}} & \left( {2l} \right)\end{matrix}$

For instance, suppose the interrogation wave is:

$\begin{matrix}{{E_{i}(t)} = {\mu_{A}{c(t)}E_{ss}{\cos\left( {\omega_{S}t} \right)}}} & (22) \\{{and}\text{:}} & \; \\{{R_{c}(\tau)} = {\left\langle {{c(t)}{c\left( {t - \tau} \right)}} \right\rangle \approx \left\{ \begin{matrix}{1;\ {\tau \approx 0}} \\{0;{\tau \neq 0}}\end{matrix} \right.}} & (23)\end{matrix}$

then a valid decoding and temporal and spatial domains demultiplexingfunction is:

$\begin{matrix}{\mspace{79mu}{{{d(t)} = {\mu_{d}{C(t)}E_{L}{\cos\left( {{\left( {{\Delta\omega} + \omega_{S}} \right)t} + \phi} \right)}}}{{b\left( {t,\tau} \right)} = {\left\langle {{d\left( {t - \tau} \right)}{E_{i}(t)}} \right\rangle = \left\{ \begin{matrix}{{\frac{\mu_{d}\mu_{a}E_{SS}E_{L}}{2}{\cos\left( {{\Delta{\omega\left( {t - \tau} \right)}} + \phi - {\omega_{s}\tau}} \right)}};{\tau = 0}} \\{0;\ {otherwise}}\end{matrix} \right.}}}} & (24)\end{matrix}$

Therefore, delaying the correlation decoding/demultiplexing functiond(t) allows demultiplexing of delay multiplexed signals identifiable byspeed of propagation and distance of flyback travel. Suppose an opticalwave is formed from a summation of delayed signals modulated onto theinterrogation wave E_(i)(t), then the received wave, E_(b)(t), is:

E _(b)(t)=Σ_(n=1) ^(N) A _(n)(t−τ _(n))μ_(A) c(t−τ _(n))E _(SS)cos(ω_(S)(t−τ _(n))+Φ_(n)(t−τ _(n)))  (25)

Then multiplying by the decoding/demultiplexing function, d(t−τ_(m)), weobtain:

$\begin{matrix}{\mspace{79mu}{{{d(t)} = {\mu_{d}{c(t)}E_{L}{\cos\left( {{\left( {{\Delta\omega} + \omega_{S}} \right)t} + \phi} \right)}}}\mspace{79mu}{{b\left( {t,\tau_{m}} \right)} = \left\langle {{d\left( {t - \tau_{m}} \right)}{E_{b}(t)}} \right\rangle}{{b\left( {t,\tau_{m}} \right)} \approx {\frac{\mu_{d}\mu_{A}E_{SS}E_{L}}{2}{A_{m}\left( {t - \tau_{m}} \right)}{\cos\left( {{\Delta{\omega\left( {t - \tau_{m}} \right)}} + \phi - {\omega_{S}\tau_{m}} + {\Phi_{m}\left( {t - \tau_{m}} \right)}} \right)}}}}} & (26)\end{matrix}$

Because τ_(m) is unique, the amplitude signal A_(m)(t−τ_(m)) and thephase signal Φ_(m)(t−τ_(m)) are both extracted from E_(b)(t) bymultiplying by the decoding/demultiplexing function, d(t−τ_(m)). Thetechnique is applicable to a wide variety of other optical-signalmultiplexing applications. Specifically, the technique can be used tospatially separate optical signals arriving from a temporally varyingtime-domain-reflectometer optical-backscatter process from an array offiber-optic acoustic sensors.

Description and Operation of the Rayleigh Optical Scattering andEncoding (ROSE) System ROSE Optical Phase Sensor Interrogation EnablesSensor Subsystem

In order to more fully describe the capabilities and features oftechniques and systems disclosed herein, the application of a system toa subsystem 16000, FIG. 16, of ROSE which launches an interrogationsignal onto fiber span 16002 and retrieves light-wave back propagation(a redirected optical signal) from a continuum of locations along thespan. Back propagation mechanisms may include Rayleigh OpticalScattering (ROS) and other effects generated within the optical fiber.Rayleigh Optical Scattering (ROS) in an optical fiber backscatters lightincident upon the fiber. The incident light transverses down the opticalfiber to the scattering point/region. At the scattering region theincident light is backscattered back up the optical fiber. As the lighttransverses the round-trip optical path (i.e., distance of flybacktravel), any disturbance of the fiber which increase or decrease theoptical path length will cause the phase of the incident andbackscattered light to be modulated. Suppose a pressure is applied tothe optical fiber. The pressure elongates the path length of the lighttraversing the region.

Refer to FIG. 16 for the following discussion. In the FIGS. like partscorrespond to like numbers. Let p(t, z) be pressure applied to theoutside of the optical fiber at time, t, and at point or length, z,along the fiber axis. Then, if an interrogation optical wave (sourceoptical signal), E_(i)(t), generated by a light source (e.g., a laser)16004, passed through a polarization-preserving optical coupler 16006and modulated by optical modulator 16008 is applied to optical coupler16010, this results in the following output interrogation wave,E_(i)(t), being transmitted down the fiber 16002:

E _(i)(t)=μ_(A) c(t)E _(ss) cos(ω_(S) t)  (27)

The backscattered wave, E_(b)(t), arriving back at the optical coupler16010 from ROSE fiber optic array 16002 passes into an optical path16012. The backscattered light (redirected optical signal) that arrivesat optical path 16012 is the summation of all light backscattered from acontinuum of locations (components of the redirected optical signal)along the length of the ROSE fiber optic span 16002. As will laterherein be described in detail, fiber 16002 has a longitudinal straincomponent enhancing coating 16014. If r(z) is the reflection density atpoint or length z along the fiber and c_(L) is the optical-wave speedwithin the fiber, then the backscattered light after a pressure p(t, z)is applied to fiber is represented mathematically as:

$\begin{matrix}{{E_{b}(t)} = {\int_{0}^{\infty}{{r\left( {\overset{\hat{}}{z}\left( {t,z} \right)} \right)}\mu_{A}{c\left( {t - \frac{2{\overset{\hat{}}{z}\left( {t,z} \right)}}{c_{L}}} \right)}E_{SS}{\cos\left( {\omega_{S}\left( {t - \frac{2{\overset{\hat{}}{z}\left( t_{Z} \right)}}{c_{L}}} \right)} \right)}d\; z}}} & (28)\end{matrix}$

where:

{circumflex over (z)}(t,z)=z+μ _(L)∫₀ ^(z) p(t,x)dx  (29)

If the distributed reflection, r(z), is essentially independent of theapplied pressure, p(t, z), then the backscatter is:

$\begin{matrix}{{E_{b}(t)} = {\int_{0}^{\infty}{{r(z)}\mu_{A}{c\left( {t - \frac{2{\overset{\hat{}}{z}\left( {t,z} \right)}}{c_{L}}} \right)}E_{SS}{\cos\left( {\omega_{S}\left( {t - \frac{2{\overset{\hat{}}{z}\left( {t,z} \right)}}{c_{L}}} \right)} \right)}d\;{z.}}}} & (30)\end{matrix}$

Since optical path length change caused by the applied pressure, p(t, z)is usually extremely small (on the order of 10⁻⁶ to 10¹ times an opticalwavelength), the backscattered light from each z distance down the fiberarrives at the optical path 16012 with a transversal delay, τ(t, z),equal to:

$\begin{matrix}{{\tau\left( {t,z} \right)} \approx \frac{2z}{c_{L}}} & (31)\end{matrix}$

Therefore, to receive the signal S₁ backscattered from the fiber regionat length-down-the-fiber z=L₁, the correlational multiplexingcharacteristic of the transmitted interrogation light (source opticalsignal) can be utilized. Multiplication of the total backscatteredoptical signal (redirected optical signal) by the correlationdecoding/demultiplexing function, d(t−τ(t, z₁)), produces an outputwhich contains the signal, S₁ (component of the redirected opticalsignal) backscattered from a distance L₁ down the fiber 16002 andrejects signals (other components of the redirected optical signal)originating from other fiber regions, such as S₂, S_(n), etc.Representing this process mathematically, the resulting channel output,B(t, L₁) is obtained as follows:

$\begin{matrix}{\begin{matrix}{{b\left( {t,\tau_{1}} \right)} = \left\langle {{d\left( {t - \tau_{1}} \right)}{E_{b}(t)}} \right\rangle} \\{= \left\langle {{d\left( {t - \frac{2L_{1}}{c_{L}}} \right)}{E_{b}(t)}} \right\rangle} \\{= {B\left( {t,\ L_{1}} \right)}}\end{matrix}\mspace{79mu}{{d\left( {t - \frac{2L_{1}}{c_{L}}} \right)} = {\mu_{d}{c\left( {c - \frac{2L_{1}}{c_{L}}} \right)}E_{L}{\cos\left( {{\left( {{\Delta\omega} + \omega_{S}} \right)\left( {c - \frac{2L_{1}}{c_{L}}} \right)} + \phi} \right)}}}\mspace{79mu}{{E_{b}(t)} = {\int_{0}^{\infty}{{r(z)}\mu_{A}{c\left( {c - \frac{2z}{c_{L}}} \right)}E_{SS}{\cos\left( {\omega_{S}\left( {c - \frac{2{\overset{\hat{}}{z}\left( {t,z} \right)}}{c_{L}}} \right)} \right)}{dz}}}}\mspace{79mu}{{\Phi\left( {z,\ L_{1}} \right)} = {\phi - \frac{2\left( {{\Delta\omega} + \omega_{S}} \right)L_{1}}{c_{L}} + {\Delta\omega\frac{2z}{c_{L}}}}}} & (32) \\{{{B\left( {t,L_{1}} \right)} = {\mu_{d}\mu_{A}E_{L}E_{SS}{\int_{0}^{\infty}{{r(z)}{R_{c}\left( \frac{2\left( {z - L_{1}} \right)}{C_{L}} \right)}{\cos\left( {{\Delta\omega t} + {\Phi\left( {z,L_{1}} \right)} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{Z}{{p\left( {t,x} \right)}d\; x}}}} \right)}z\; d\; z}}}}\mspace{79mu}{{{\Delta\Phi}\left( {t,z} \right)} = {{\Phi\left( {z,L_{1}} \right)} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{z}{{p\left( {t,x} \right)}d\; x}}}}}\mspace{79mu}{{B\left( {t,L_{1}} \right)} = {v_{E}{\int_{0}^{\infty}{{r(z)}{R_{c}\left( \frac{2\left( {z - L_{1}} \right)}{C_{L}} \right)}{\cos\left( {{\Delta\omega t} + {\Delta{\Phi\left( {t,z} \right)}}} \right)}d\; z}}}}\mspace{79mu}{{B\left( {t,L_{1}} \right)} = {V_{E}r_{L_{1}}{\cos\left( {{\Delta\omega t} + \Phi_{L_{1}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{1}}{{p\left( {t,x} \right)}d\; x}}}} \right)}}}} & (33)\end{matrix}$

Because of the correlation properties of the interrogation light(modulated source optical signal), the autocorrelation function R_(c)(τ)is very small at all spatial locations except those in the vicinity ofz=L₁. Therefore, all signals (other components of the redirected opticalsignal) originating anywhere else are rejected. Furthermore, the phaseof the channel output at location L₁ will be the summation orintegration of all pressure changes along the bi-directional transversalpath. This unusual phenomenon has been demonstrated with experimentalhardware.

Once the correlation process isolates the optical signal originatingfrom a spatial region, the signal is phase demodulated to extract thepressure information. The signal is I(in phase) and Q (quadrature phase)demodulated as follows:

$\begin{matrix}{\mspace{79mu}{{{B_{1}\left( {t,L_{1}} \right)} = \left\langle {{B\left( {t,L_{1}} \right)}{\cos\left( {\Delta\omega t} \right)}} \right\rangle}{{{B_{1}\left( {t,L_{1}} \right)} \approx {V_{E}r_{L_{1}}{\cos\left( {\Phi_{L_{1}} + \frac{2\mu_{L}\omega_{S}}{c_{L}}} \right)}{\int_{0}^{L_{1}}{{p\left( {t,x} \right)}d\; x}}}} = {V_{1}{\cos\left( \phi_{1} \right)}}}\mspace{79mu}{{B_{Q}\left( {t,L_{1}} \right)} = \left\langle {{B\left( {t,L_{1}} \right)}{\sin\left( {\Delta\omega t} \right)}} \right\rangle}{{{B_{Q}\left( {t,L_{1}} \right)} \approx {{- V_{E}}r_{1}{\sin\left( {\Phi_{L_{1}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{1}}{{p\left( {t,x} \right)}d\; x}}}} \right)}}} = {{- V_{1}}{\sin\left( \Phi_{1} \right)}}}}} & (34)\end{matrix}$

Then I & Q, or cosine phase and sine phase, outputs are converted intoeither phase rate or phase outputs with simple analog or digitalhardware. The phase, so demodulated, allows the inference of informationabout the acoustic pressure down the fiber to the measurement point.

Once the I& Q outputs are generated, the temporal phase state of B(t,L₁) can be determined by one of several types of phase-demodulationprocesses. The phase state of the region of L₁ spatial delay istherefore:

$\begin{matrix}{\Phi_{1} = {\Phi_{L1} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{1}}{{p\left( {t,x} \right)}d\; x}}}}} & (35)\end{matrix}$

Likewise, the plurality (which may be a large number, e.g., 5000) ofoptical signals (components of the redirected optical signal) arisingwith spatial delays, such as the propagation time for flyback travel toL₂ or L_(n), can be correlated out of the backscattered signal(redirected optical signal) E_(b)(t). These are:

$\begin{matrix}{{{B\left( {t,L_{2}} \right)} \approx {V_{E}r_{L_{2}}{\cos\left( {{\Delta\omega t} + \Phi_{L_{2}} + {\frac{2\mu_{L}\omega_{S}}{C_{L}}{\int_{0}^{L_{2}}{{p\left( {t,x} \right)}d\; x}}}} \right)}}}{{B\left( {t,L_{n}} \right)} \approx {V_{E}r_{L_{n}}{\cos\left( {{\Delta\omega t} + \phi_{L_{n}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{2}}{{p\left( {t,x} \right)}d\; x}}}} \right)}}}} & (36)\end{matrix}$

With corresponding phase signals of:

$\begin{matrix}{{\Phi_{2} = {\Phi_{L_{2}} + {\frac{2\mu_{L}\omega_{S}}{C_{L}}{\int_{0}^{L_{2}}{{p\left( {t,x} \right)}d\; x}}}}}{\Phi_{n} = {\Phi_{L_{n}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{n}}{{p\left( {t,x} \right)}d\;{x.}}}}}}} & (37)\end{matrix}$

The phase signals, obtained by phase demodulation of each B(t, L_(m)),represent a pressure field p(t, z) which is integrated along the length,z, of the fiber. Therefore, rather than directly measure p(t, z) thesensor provides all of the accumulated pressure effects down the fiberto the measurement point, L_(m) (where m is an integer corresponding tothe measurement point). In sensor arrays, it is usually desired todetect the pressure over a specific measurement region. If twocomponents S_(j) and S_(k) of the redirected optical signal are receivedfrom measurement lengths L_(j) and L_(k), the corresponding demodulatedphases Φ_(j) and Φ_(k) are:

$\begin{matrix}{{\Phi_{j} = {\Phi_{L_{j}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{j}}{{p\left( {t,x} \right)}d\; x}}}}}{\Phi_{k} = {\Phi_{L_{k}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{k}}{{p\left( {t,x} \right)}d\; x}}}}}} & (38)\end{matrix}$

A sensor between the lengths down the fiber of L_(j) and L_(k)(L_(k)>L_(j)) is formed by subtracting the two phases:

$\begin{matrix}{{{\Phi_{k} - \Phi_{j} - {\Delta\Phi_{kj}}} = {\left( {\Phi_{L_{k}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{k}}{{p\left( {t,x} \right)}d\; x}}}} \right) - \left( {\Phi_{L_{1}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{0}^{L_{j}}{{p\left( {t,x} \right)}d\; x}}}} \right)}}{{\Delta\Phi_{kj}} = {\Phi_{L_{k}} - \Phi_{L_{j}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}\left( {{\int_{0}^{L_{k}}{{p\left( {t,x} \right)}d\; x}} - {\int_{0}^{L_{j}}{{p\left( {t,x} \right)}d\; x}}} \right)}}}\mspace{79mu}{{\Delta\Phi}_{kj} = {\Phi_{L_{k}} - \Phi_{L_{j}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{L_{j}}^{L_{k}}{{p\left( {t,x} \right)}d\; x}}}}}\mspace{79mu}{{\Delta\Phi}_{kj} = {{\Delta\Phi_{L_{k}L_{j}}} + {\frac{2\mu_{L}\omega_{S}}{c_{L}}{\int_{L_{j}}^{L_{k}}{{p\left( {t,x} \right)}d\; x}}}}}} & (39)\end{matrix}$

The resulting sensor is of length ΔL=L_(k)−L_(j) with a center positionof (L_(k)+L_(j))/2. The differencing of phase signals Φ_(j) and Φ_(k)into a new phase signal ΔΦ_(kj), allows a virtual sensor of arbitraryposition and length to be formed. The resulting spatially differentialsensor also adds the advantage of minimizing other effects such aslead-in fiber strum or vibration, which create unwanted phase signals.

The above phenomena illustrate that when the interrogation light isproperly encoded, a ROSE (Rayleigh Optical Scattering and Encoding)sensor system is enabled. The techniques, components, systems, andfeatures disclosed herein, therefore, enable the ROSE concept. Anembodiment of a system disclosed herein enables spatial discriminationof the optical backscatter effects in a ROSE sensor. Thespatial-differencing technique rejects unwanted common mode signalsinadvertently introduced in optical-fiber leads down to the sensorregion. Such a technique also applies in a similar manner to moreconventional fiber-optic acoustic-sensor arrays (i.e., those havingBragg reflective grating sensors) or to non-fiber optic remote opticalsensors that detect phase.

Pointwise Signal Delay Multiplexing

Embodiments described herein also apply to point-wise non-distributedsensors or artificially generated multiplexing by electronics means. Theinterrogation light wave (modulated source optical signal) can beintercepted and retransmitted back to the receiver with an artificial,electronically generated delay, as a means of delay/correlationmultiplexing many channels.

Description of a Fiber System Implementation

Embodiments can be realized with bulk-optical, fiber-optical orintegrated-optical components. For simplicity, a fiber-opticimplementation will be presented. However, the fiber-optic embodiment isbeing presented without intent of limitation. The teachings herein canbe used to implement a reflectometer system using the described andother instrumentalities providing a light path that has the innateproperty of producing back propagation of portions (components) of aninterrogation signal (source optical signal) at a continuum of locationsalong the length of the propagation path therethrough.

Optical Transmitter and Time-Delay Multiplexing Process

FIG. 16 is an illustrative block diagram implementation of the Rayleighoptical scattering and encoding (ROSE) sensor system 16016, which canbe, or can be similar to, the system 1000 of FIG. 1, and all componentsbut the optical fiber 16002 can be, or can be the same as, theelectro-opto subsystem 1002 of FIG. 1. Like parts correspond to likenumbers throughout the figures. A light wave (optical signal) fromtransmitter laser (optical source) 16004 is propagated throughpolarization-preserving optical coupler or beam splitter 16006. Thesmaller portion of the transmitter laser power split off by the opticalcoupler 16006 is passed by an optical path 16018 to an acousto-opticmodulator 16020. The larger portion of the transmitter laser light poweris split by optical coupler 16006 and propagated to an electro-opticalmodulator 16008. The electro-optical modulator 16008 modulates the laserlight passing from the optical coupler 16006 with a correlation codec(t) as electronically generated in a master correlation-code generator16022, and which may be amplified by an amplifier (not shown in FIG.16). The correlation code c(t) is modulated onto the laser light (i.e.,modulates the laser light) by the action of the electro-opticalmodulator, 16008. This modulated light comprises the opticalinterrogation light wave (modulated source optical signal) E_(i)(t). Theelectro-optical modulator 16008 may modulate the amplitude,polarization, or phase of the laser light (to modulate the sourceoptical signal, and, therefore, generate the modulated source opticalsignal) subject to the teachings herein. The interrogation light wave(modulated source optical signal) is propagated from the electro-opticalmodulator 16008 to an optical coupler, beam splitter, or circulator16010. The interrogation light wave (modulated source optical signal)passes through the optical coupler 16010 into the optical fiber 16002,or another light-propagation medium (not shown in FIG. 16). Hereinafter,“down”, indicates a transversal on the optical path 16002 away from thecoupler 16010; “up” indicates a transversal on the optical path 16002toward the optical coupler, beam splitter, or circulator 16010.

The interrogation light wave (modulated source optical signal) whichtransverses down the optical fiber or other optical medium 16002 ismodulated and is backscattered or returned by other means withequivalent optical path lengths (each equivalent to a time delay), L₁,L₂, . . . , L_(n) corresponding to sensors, multiplexed channels, orzones S₁, S₂, . . . , S_(n). The returned interrogation light wave(redirected optical signal) is a composite optical signal modulated bysignals (components of the redirected optical signal) due to the S₁through S_(n) modulating and time-domain multiplexing actions.

More particularly, the propagation of the optical spread-spectruminterrogation signal (modulated source optical signal) down thecontinuous full span of the optical fiber 16002, signal launch end toremote end, causes a back-propagating composite optical signal(redirected optical signal), which is the linear summation, orintegration spatially, of all of the individual, continuous, orcontinuum of back-reflections (components of the redirected opticalsignal) along the entire span of the optical fiber.

One component of this composite signal (redirected optical signal) iscomprised of the naturally occurring continuum of optical backreflections (components of the redirected optical signal includingRayleigh optical scattering ((ROS)) effects) of the opticalspread-spectrum carrier signal (modulated source optical signal) that isformed by modulating the primary carrier signal by thespectrum-spreading signals. Another component is comprised of theartificially occurring optical back reflections, either-point wisereflections or distributed reflections, of the optical spread-spectrumcarrier signal that is formed due to propagation discontinuities as theresult of presence of one or more fiber cable couplers along the opticalfiber 16002. Still another component comprised of the continuum ofmodulations at locations along the span of the reflected signals due tolongitudinal components of optical-path-length change, causing a delayin the reflected signal, experienced by the optical fiber 16002 alongits length.

Such optical-path-length change or delay may be caused by a variety ofpossible sources including acoustic pressure waves incident to the fiber16002, electromagnetic fields coupled to the fiber, mechanical strain orpressure on the fiber, thermal strain or pressure induced in the fiber,or other means of causing change in the optical path length. Use of theacoustic-pressure-waves mode of changing path length in perimeterintrusion monitoring systems is the principle, yet nonlimiting,embodiment illustrated herein. In this use, optical-fiber span 16002 isemployed to provide an array of virtual geophones buried at a range ofdepths beneath the surface of the ground of about between six (6) inchesand one (1) foot, to sense motion of an object on the surface of theground. The acoustic-pressure-wave sensing mode is also useful to senseseismic signals, as for example as linear arrays inserted into a casingstructure of an existing oil well. Predeterminedartificial-pressure-wave-producing shocks are imparted into the ground,and the responses from the sensor are used to locate secondary oildeposits. The acoustic-pressure-wave sensing mode is further useful foremploying the span of the optical fiber 16002 as an array of virtualhydrophones, with the media that couples the signals to the hydrophonesat least in part being the body of water in which the array is immersed.Such hydrophone arrays find use as naval-undersea-warfare towed arrays,or towed geophysical-exploration arrays. In the latter, the arraysrespond to artificially produced shocks of predetermined character andlocation induced in the body of water, and the responses of the array tobottom return signals are used to locate ocean bottom geophysicalfeatures indicating likely presence of an oil deposit. Yet further, asensing position (zone) on the fiber 16002 could be used to receive, asan input, microphonic signals suitably imparted to the region of thesensing position. The electromagnetic field sensing mode of the opticalfiber 16002 could be used for monitoring electronic signals along atelecommunication cable's span to localize malfunctions. Responses ofthe optical fiber 16002 to mechanical, pressure, or thermal strains canbe used in systems for monitoring such strains.

The composite light wave (redirected optical signal) propagates up theoptical fiber or medium 16002, passes through optical coupler, beamsplitter or circulator, 16010, to optical pathway 16012, which passesthe backscattered, time-delay multiplexed, composite light wave(redirected optical signal) E_(b)(t) to an optical receiver 16024 via anoptical polarization splitter 16026. Although not shown in FIG. 16, thesystem 16016 can include an optical isolator disposed between thecirculator 16010 and the polarization splitter 16026 to preventreflections of the redirected optical signal by the polarizationsplitter from propagating through the circulator andpolarization-preserving coupler 16006 to the light source (e.g., laser)16004.

In an embodiment, fiber 16002 is of the relatively low-cost,conventional single-mode or multimode, fiber-cable type.

Further, in an embodiment, the fiber 16002 has extruded thereon acoating 16014 of a material which enhances the longitudinal strain whichthe fiber undergoes from a given radially, or generally laterally,applied pressure-wave strain. Materials that provide such enhancementinclude extrudable thermoplastic polymers (TPU's) or extrudablethermoplastic elastomers (TPE's) that exhibit a combination of a lowYoung's modules (E) and a low Poisson's ratio (a). The Poisson's ratiois, for example, below 0.5, which is the Poisson's ratio of naturalrubber. Examples of such materials include: (i) low-densitypolyethylene, having characteristic E=1.31 dynes/cm²×10⁻¹⁰ and σ=0.445;and (ii) polystyrene, E=3.78 dynes/cm² and σ=0.35 (values as reported inthe paper, R. Hughes and J. Javzynski, Static Pressure SensitivityAmplification in Interferometric Fiber-Optic Hydrophones, AppliedOptical/Vol. 19/No. 1/1 Jan. 1980, which is incorporated herein byreference).

An alternate embodiment of fiber 16002, albeit involving significantlygreater cost per unit length of the fiber, is a fiber in themore-expensive form of a polarization-preserving, orsingle-polarization, optical fiber. The polarization-preserving fiber ofthis type holds the backscattering light (redirected optical signal) ina narrow range of polarization states so that a substantially single RFsignal 16028 enters a single set of correlators 16030, reducing thecomplexity of the system 16016. But in cases involving long surveillancefibers, this alternative embodiment becomes expensive in cost of thefiber 16002. In such an embodiment, a polarization splitter 16026 isincluded to allow processing of the two polarization components of theredirected optical signal.

The correlation code generator 16022 generates a signal c(t) that has abroad bandwidth. The broadband nature of the correlation code is allowsobtaining the desired properties in the signal's autocorrelationfunction. The calculation and definition of the autocorrelation functionof any general signal is well known and defined in availablesignal-processing literature. The correlation code signal c(t) is sostructured that its autocorrelation function is highly peaked at zerodelay, and is very small away from zero delay. This criterion is wellknown to those of skill in the art and is the essence of why thecorrelation code has a broad bandwidth. Any signal that has the desiredautocorrelation function properties can be used as the correlation codein the embodiments described herein. There are many reasons for choosingone correlation code over another: ease of creation; autocorrelationproperties; cost of creation hardware; cost of correlation hardware; andeffectiveness in producing spread-spectrum signal effects. According tothe teachings herein, the correlation code for disclosed embodiments canbe a binary sequence with a desired transorthogonal autocorrelationproperty (sometimes called a pseudonoise sequence), a pseudorandomnumber (PRN) sequence with the such desired autocorrelation property,chirps, or other types of signals that provide correlation codes havingpredicable non-repetitive behavior. The foregoing list of types ofsequence signals that may be employed to modulate the carrier light-wavesignal (source optical signal) includes both “binary pseudonoisesequences” and “pseudorandom number (PRN) sequences.” For purposes ofconstruction of this specification, these terms are employed as they aredefined under the listings “Pseudonoise (PN) sequence (communicationsatellite)” and “Pseudorandom number sequence” at pages 747 and 748 ofthe “IEEE Standard Dictionary of Electrical and Electronic Terms”(Fourth Edition), which listings are incorporated herein by reference.Further for purposes of construction of this specification, it is deemedthat “binary pseudonoise sequence” is generic and “pseudorandom numbersequence” is a species thereof. Still further for purposes ofconstruction of this specification, both terms are deemed to includeanalog-signal forms of sequences as well as digital-signal forms.

It is to be appreciated that in addition to its correlation-encodingfunction, master-correlating-code generator 16022 is a source of aspectrum-spreading signal comprised of a spectrum-spreading signal thatproduces an autocorrelation that is well behaved. It has one dominantmaxima at zero correlation delay, and its spectrum is large enough toprovide sampling of said optical fiber 16002 spatially along the lengthof the fiber with a resolution commensurate with a sub-length ΔZ of thefiber. These characteristics enable segmentation of the optical fiber16002 of span length L into n segments in accordance with arelationship:

L<ΔZ·n  (40)

In this relationship, ΔZ is a segment length of a span of the fiber16002 whose length is one-half the distance traveled by lightpropagating through one zero delay temporal time span of theautocorrelation maxima, ΔT, such that c_(L) is the speed of light in theoptical fiber and ΔT is approximately equal to the reciprocal of thespread-signal optical bandwidth.

An illustrative embodiment of the code generator 16022 is ashift-register-type pseudorandom-number-code generator, having n bits,wherein a code is generated that satisfies saidresolution-sublength-and-segment-length relationship by choosing anappropriate combination of the number of its bits and the clocktime/rate.

The temporal length of the code sequence that is reiteratively producedby the generator 16022 may be either less than the time period forpropagation of a light wave to the remote end of fiber 16002 andpropagation back of a backscattering redirected optical signal (i.e.,the distance of flyback travel), or greater than this time period. Itcannot be equal to this period, or else ambiguity may occur.

The predetermined timing base employed by the source of thespectrum-spreading signals, which timing base determines the length of aΔZ segment, is so chosen to provide a positive enhancement to the ratioof the power of back-propagating Rayleigh scattering effect PR to thepower of the forward-propagated Rayleigh scattering effect P_(T), inaccordance with the following equation:

$\begin{matrix}{{\frac{P_{r}}{P_{t}}\left\lbrack {dB} \right\rbrack} = {{{- 7}0} + {10{\log_{10}\left( {\Delta L} \right)}} - \frac{\Delta Z}{100}}} & (41)\end{matrix}$

Local Oscillator Generation

Still referring to FIG. 16, the fiber-optic coupler or beam splitter16006, which is normally a polarization-preserving device, passes aportion of the laser beam (source optical signal) from the laser (lightsource) 16000 to an acousto-opto modulator 16032; typically the power ofthe portion of the laser beam that the splitter 16006 passes to theacousto-opto modulator is significantly less than the power of theportion of the laser beam that the splitter passes to optical fiber16002 via the electro-optic modulator 16008 and the circulator 16010.The acousto-optic modulator 16032 upshifts the frequency of the passedportion of the laser beam by an intermediate frequency, for example,approximately 900 MHz. Furthermore, where the system 16016 is configuredto process both polarization components of the redirected optical signalfrom the fiber 16002, an optical polarization-preserving splitter 16033splits the polarized frequency-upshifted optical signal from afirst-order output of the acousto-optic modulator 16032 into twopolarized beams of the frequency-upshifted source optical signal.Optical pathway 16034 propagates the upshifted beams from thepolarization-preserving optical splitter 16033 to the optical receiver(signal mixer) 16024. The frequency-upshifted beams on the opticalpathway 16034, which couples the frequency-upshifted beams to theoptical-receiver assembly 16024, are aligned to have the samepolarizations as the composite-light-wave polarization components,respectively, on the optical pathway 16038. Furthermore, although notshown, the acousto-optic modulator 16032 may also generate azeroth-order output signal that the system 16016 can use, for example,to determine whether the light source 16004 is generating the sourceoptical signal (e.g., to determine whether the light source is “on”), oras an input to a feedback circuit configured to maintain the power ofthe source optical signal at a desired level.

Still referring to FIG. 16, each of the composite light-wavepolarization components on the optical path 16038 is an input into theoptical receiver 16024. The polarization components of the localoscillator light wave on the optical path 16034 are also each input tothe optical receiver 16024. The local-oscillator andcomposite-light-wave polarization components are respectively interferedon photodetectors, thus producing an electronic signal whichelectronically represents the heterodyned optical interference powerbetween the two light waves. The resulting composite radio-frequencypolarization signal components on the respective pathways of the path16036 represent, electronically, the composite light-wave signalcomponents on the pathways of the optical path 16038. The compositepolarization-component electronic receiver signals are passed from thepath 16036 to the correlator circuit 16030. The local-oscillatorlight-wave polarization components on optical path 16034 are eachinterfered with the composite light wave polarization components onoptical path 16036. The interference powers are photo detected inoptical receiver 16024 by optically interfering each polarizationcomponent of the composite back propagating light wave (redirectedoptical signal) on the respective polarization component of the localoscillator signal. As one of the components of this interfering action,there is produced, for each polarization component, a respectivedifference beat signal that is a composite radio frequencyrepresentation of the corresponding component of the composite lightwave on optical path 16038.

This interfering of the polarization components of the local oscillatoroutput light wave 16034 and the polarization components of the compositeback-propagating CW light wave (redirected optical signal) 16038provides the translation of the polarization components from the opticaldomain to respective CW radio frequency (r.f.) composite difference beatsignals 16036, one respective beat signal per polarization component.This reduces the frequencies of the optical polarization componentsinput to the photodiode mixer 16024 into an electronically processablesignal frequency range. It is to be appreciated that the r.f. compositedifference signals produce by this translation action includes havingcounterpart components of the aforesaid components of the compositebackpropagating light-wave signal, with the phase states of thesecounterpart r.f. domain signals the same as the phase states of thecorresponding components of the back-propagating light wave (theredirected optical signal.

Where a signal-polarization fiber 16010 is used, the polarizationsplitter 16026 may be omitted from the system 16016, and theacousto-optic modulator 16032 can be configured to output only a singleupshifted optical local oscillator signal. This would result in theoptical mixer 16024 having only two inputs and a single output. Forexample purposes, hereinafter it is assumed that the optical mixer 16024outputs only a single signal, it being understood that in an embodimentin which the optical mixer outputs two polarization components insteadof a single signal, the processing steps described below are performedseparately on each of the two polarization components.

In accordance with an embodiment, laser 16000 is to have sufficientlystringent high-performance capability with respect to exactness offrequency to enable interference effects therebetween and heterodynedetection of acoustic perturbation signals incident to fiber 16002 toproduce beat frequencies within the radio frequency (r.f.) range. Alsoin accordance with an embodiment, laser 16000 has stringent performancecriteria with respect to the phase stability, or coherence, of its beam.The beam is to be substantially coherent over at least a propagationpath distance substantially equal to twice the length L of sensing fiber16002. For example, a commercially available non-planar, ring laser(e.g., Lightwave Electronics Corp. Model 125) would be suitable for anintruder-sensing perimeter-intrusion-monitoring fiber 16002 having alength of 8.0 km (approximately 5 miles). The laser beam of thiscommercially available laser, which is in the near infrared range, has afrequency of 227 terahertz, or a 1319 nanometer wavelength, and has afrequency stability accurate to within one part in a billion over a 1millisecond period, or 5 KHz in a 1 millisecond period.

In a preceding section, there is a description of “non-zero Δω” and amathematical demonstration of its importance in the heterodyne mode ofinterfering. It makes it possible to use relatively simple processes toavoid fading. By way of contrast, fading with the “zero Δω” homodynemode of interfering typically would entail much more difficult and lesseffective fade-avoidance processes.

Correlation Time-Delay Demultiplexing

Still referring to FIG. 16, the composite radio frequency signal onelectrical path 16036 is input into the correlator circuitry 16030. Thecorrelator circuit delays the master-correlation-code-generator output16040 an appropriate amount and correlates the delayed correlation codewith the composite radio frequency signal. This produces electricaloutputs O₁, O₂, . . . , O_(n) corresponding to signals S₁, S₂, . . . ,S_(n), in turn corresponding to spatial delays L₁, L₂, . . . , L_(n).The spatial delays L₁, L₂, . . . , L_(n) are arbitrary and programmable.The electrical output O₁ corresponds to B(t, L₁) referred to in apreceding subsection.

The correlation process is well understood in the literature. The signalthat represents the backscattered optical wave (redirected opticalsignal) in fiber array 16002 that is passed from the optical receiver16024 to the correlator circuit 16030 contains all of the informationfor all sensors or channels S₁, S₂, . . . , S_(n) at once on theelectronic signal path 16036 entering the correlator. Because thebackscattered composite signal (redirected optical signal) is modulatedwith the correlation code by electro-optic modulator 16008, thebackscattered light is time structured with the time structure of thecorrelation code. Because the correlation code is selected to havespecial autocorrelation code properties, the time structure of thecorrelation codes allows an electronic representation of thebackscattered light at positions L₁, L₂, . . . , L_(n) to be obtainedvia the correlation process in the correlator 16030. In an embodiment,the master-code generator 16022 is a shift-register typepseudorandom-number (PRN) code generator and each correlator of thecorrelator circuit 16030 is a correlation-type demodulator herein laterdescribed in greater depth. Code generator 16022 may alternatively beembodied as a binary sequence having transorthogonal autocorrelationproperties (binary pseudonoise sequence) and each correlator of thecorrelator circuit would then be a correlation-type demodulator fordemodulating a binary pseudonoise sequence, whose implementation wouldbe understood by those of skill in the art. The correlator circuit 16030uses the reference correlation code from correlation-code generator16022, which correlation code is passed via electronic path 16040 to thecorrelator circuit, as a “golden ruler” enabling sorting out by temporaland spatial domain demultiplexing electronic representations of thebackscattering optical signals (components of the redirected opticalsignal) at sensors or channels S₁, S₂, . . . , S_(n). Various delayedversions of the correlation code are multiplied by the composite signalwith all of the sensor or channel signals present simultaneously, fromelectronic path 16036, so that the electronic representations of thesensors or channels S₁, S₂, . . . , S_(n) are output from the correlatorcircuit 16030 as signals O₁, O₂, . . . , O_(n) with respect to theindex.

Correlator circuit 16030 is an electronic spread-spectrum signalde-spreader and temporal and spatial domain de-multiplexer of the r.f.signal counterpart to the optical composite signal. Its input is coupledto the output 16036 of the heterodyner and photodetector 16024, and itis operative in cooperation with said source of spectrum-spreadingsignals to perform a coherent signal-correlation process upon the r.f.counterparts of the aforesaid “one” and the aforesaid “still another”components of the composite back-propagating CW light wave (redirectedoptical signal). This causes the de-spreading of the r.f counterpart ofthe optical reflected (redirected) spread-spectrum signal and causes thetemporal and spatial demultiplexing of the r.f. counterpart of the“still another” component of the composite r.f. signal. This processingprovides signals which temporally and spatially sort the said “stillanother” component into n virtual sensor signal channels, or statedanother way n of each of the ΔZ length measurement regions, measuringthe induced optical path change at each of the n ΔZ-length segments ofthe optical fiber span 16002.

It will be appreciated that this sorting process is accomplished by theautocorrelation properties of the spectrum-spreading signal and by thetime of flight of the optical spectrum-spreading signal down to eachn^(th) reflection segment and back to the heterodyne optical receiver16024. A delayed replica of the spectrum-spreading signal is correlatedagainst the r.f. signal counterpart of the optical compositeback-propagating signal, thereby segmenting the optical fiber into nindependent segments, or virtual sensors, via the time of flight of theoptical composite back-propagating signal and the autocorrelationfunction of the transmitted spectrum-spreading signal.

It is to be appreciated that system 16016 is operating in thespread-spectrum-transmission-and-reception mode. Namely, by providingoptical interrogation light wave E₁(t) with modulation by thecorrelation code c(t), the continuous-wave carrier signal is temporallystructured into a spread-spectrum interrogation light wave thatcontinuously reiterates autocorrelatable code sequences. Then after thecorrelation subsystem 16030 provides an appropriate time of delay, thecorrelator subsystem correlates the backscattered light wave E_(b)(t)with the same output c(t) of the code generator 16022, de-spreading thespread-spectrum signal.

In accordance with well-known communication electronics theory, this hasthe effect of increasing signal output of the ROSE sensor system whilethe noise bandwidth remains the same. In temporally and spatiallysorting the r.f. counterpart of the aforesaid “still another” componentof the composite back-propagation light wave (redirected opticalsignal), the aforesaid “another” component of undesired noises, such asreflections from couplers in fiber 16002, are materially attenuated.

More particularity, in accordance with this well-known theory, thesignal-to-noise ratio (SNR) is enhanced by considerable attenuation ofnoise mechanisms in frequency ranges outside of the center-frequencylobe of the autocorrelation function and outside the pair of first sidelobes to one and the other side of the center-frequency lobe.

An illustrative embodiment of electronic spread-spectrum signalde-spreader and spatial de-multiplexer for cooperation with thepreviously described shift-register type PRN code generator may comprisea series of n-like shift-register code generators respectively receivingthe spectrum-spreading signal through a corresponding series of n feedchannels that cause delays that incrementally increase by an amount oftime bearing a predetermined relationship to the fiber-span length, andCL, the speed of light through the fiber. The composite r.f. signal isfed to a corresponding series of n multipliers connected to receive asthe other multiplier the codes generated by the respective de-spreaderand demultiplexer to thereby provide the de-spread and de-multiplexedsignal.

Heterodyne Phase Demodulation

Still referring to FIG. 16, after the composite radio-frequency signalon electrical path 16036 is correlation-time-delay demultiplexed by thecorrelator system 16030, the plurality (which upwardly may include avery large number, for instance 5,000) of outputs O₁, O₂, . . . , O_(n),on the plurality of electrical paths 16042, 16044, and 16046respectively are phase demodulated by a plurality of individual phasedemodulations in the phase-demodulator circuit 16048 (the electricalpaths 16042, 16044, and 16046 are shown in pairs, one path for eachpolarization component; but for a system in which there is a singleoutput 16028 from the optical mixer 16024, each of the electrical paths16042, 16044, and 16046 are single paths). The outputs of thephase-demodulator circuit 16048 are the corresponding plurality ofelectrical paths 16050, 16052, and 16054. The phase-demodulator outputs16050, 16052, and 16054 correspond to the correlator outputs (O₁, O₂, .. . , O_(n)) 16042, 16044, and 16046 respectively, and to thecorresponding plurality of corresponding signals S₁, S₂, . . . , S_(n)respectively corresponding to spatial delays L₁, L₂, . . . , L_(n),respectively. The outputs 16050, 16052, and 16054 electronicallyindicate (with tens of kilohertz potential bandwidth) the phase statesof optical signals S₁, S₂, . . . , S_(n). In particular, output 16050 isproportional to the temporal phase Φ_(I) of B(t, L₁) hereinbeforediscussed. The phase demodulator outputs 16052 and 16054 indicate thetemporal phase states Φ₂ and Φ_(n) of B(t, L₂) and B(t, L_(n))respectively.

Fading Free Polarization Processing

System 16016 may further include polarization-signal-characteristicprocessing functions (described above), which are used together with thepreviously described feature that the heterodyning function provides inreducing fading, of the backscattering signal, E_(b)(t). Thesepolarization processing functions are disclosed in U.S. Pat. No.6,043,921 entitled “Fading-Free Optical Phase Rate Receiver,” herebyincorporated herein in its entirely. The optical heterodyning featurethat provides benefit in reducing fading includes: (i) laser 16004 inthe formation of the optical interrogation light wave (source opticalsignal) E_(i)(t), applied to optical fiber 16002, or other linearlyextended light propagation medium producing Rayleigh effectsbackscattering, and (ii) the manipulation of this by optical receiver16024 to provide the composite electronic receive signal as opticalreceiver output 16036. This takes advantage of the feature of morefavorable heterodyne fading conditions in a way, in which polarizationand phase-state signal fading is materially reduced in the detectedbackscattered light wave E_(b)(t). The electronic decoding module 700 ofU.S. Pat. No. 6,043,921 is substantially an equivalent to the correlatorsystem 16030 herein. However, the system disclosed in U.S. Pat. No.6,043,921 for implementing polarization fading reduction (if notsubstantially eliminating fading) is a generalized stand-alone systemfor processing any optical phase signal having temporally varyingpolarization, phase, and phase frequency. It must be adapted forapplication to system 16016 by appropriate integration into system 16016including the two following alternative approaches.

One approach for such adaptation passes the fade-free optical phase rate(FFOPR) photoreceiver RF signal to the correlator 16030, performs thecorrelation on the RF signal and completes the Phase Demodulation by Inphase and Quadrature phase (hereinafter I& Q) demodulating thecorrelated RF signal into outputs. This method creates low bandwidth I&Q components and therefore uses low-bandwidth analog-to-digitalconverters (implying a use of a large number of analog RF correlationelectronic components). This RF correlator approach uses two correlatorcircuits for every virtual sensor element, or spatial channel, alongfiber 16002. One correlator is used for the vertical polarization RFsignal path and one correlator is used for the horizontal polarizationRF signal path (i.e., the two polarization components described above).

Another approach applies the I& Q demodulator of FIG. 7 of the U.S. Pat.No. 6,043,921 prior to correlation. This approach, therefore, correlatesa wideband set of four I& Q signals. One I, Q, set is for horizontalpolarization and the other I, Q, set is for the vertical polarization.In this case the I& Q signals are the I& Q signals for the whole virtualarray rather than for one virtual sensor element of the array. Fourcorrelators are used for each sensor element. One correlator is appliedto each of the four wide bandwidth I& Q signals for each virtual sensorelement. This second approach users very wideband analog-to-digitalconverters, but allows digital correlators to be used instead of analogRF correlators. The RF correlator or first approach uses far moreanalog-to-digital converters and RF electronics. The digital-correlatorapproach enables the correlators to be implemented by the digitalapproaches of massively integrated logic circuits and/or programmedprocessors, typically requiring far more digital logic, butsubstantially reducing the r.f. electronics and number ofanalog-to-digital converter units in the system.

Phase Differencing

Still referring to FIG. 16, the plurality (which upwardly may include avery large number, for instance 5,000) of signals indicating the phasestates Φ₁, Φ₂, . . . , Φ_(n) on electrical paths 16050, 16052, and16054, respectively, are input into a phase-differencer circuit 16060.The phase differencer 16060 forms a corresponding plurality of outputs16062, 16064, and 16066, which are arbitrarily and programmably assignedas the subtractions of any two pairs of phase signals Φ_(j) and Φ_(k)(where j and k are selected from 1, 2, . . . , n).

Each of the programmably selectable pairs of differenced phase signalsform a signal ΔΦ_(kj), which is spatially bounded within the region ofthe fiber between lengths L_(j) and L_(k). The phase differencer 16060,therefore, produces differential phase outputs corresponding to a set ofprogrammable-length and -position virtual sensors.

Stated another way, each programmable selection of pairs of phasesignals forms a virtual spatial differential sensor that senses thedifference between the phases of the Δω output of the photodetectorsub-system (which is the subject of the next subsections) in receiver16024. Each Δω is an r.f. difference beat signal representative of theaforesaid “still another” component of the composite back-propagating CWlight-wave signal (redirected optical signal), which passes from thelaunch end of fiber span 16002 to directional coupler 16010. Thesesignals from each pair therefore represent signals of virtual spatialdifferential sensors along fiber span 16002. As a result of the choiceof pairs being selectively programmable, these virtual sensors can beemployed to implement adaptive apertures in processing signals incidenton the fiber span 16002. This feature would be useful, for example, inenabling security-system operators to classify objects causing acousticpressure-wave signals incident up a fiber span 16002 used as aperimeter-intrusion monitoring line.

A signal analyzer 16068 is configured to detect and to analyze arecovered acoustic signal in response to the optical phase (from thephase demodulator 16048) of the component of the redirected opticalsignal from the corresponding fiber zone, and in response to anoptical-phase difference (from the phase difference 16060) between theoptical phase of the component of the redirected optical signal from thecorresponding fiber zone and an optical phase of a component of theredirected optical signal from another fiber zone. For example, thesignal analyzer 16068 can be configured to determine a frequency contentof the recovered acoustic signal by implementing, e.g., an FFT, thereon.And the signal analyzer 16068 can be configured to determine a locationon which the acoustic signal is incident upon the optical fiber 16002 inresponse to the optical-phase difference. Furthermore, the signalanalyzer 16068 can be configured to detect a presence of the acousticsignal in response to the optical phase from the phase demodulator 16048changing over time.

And a signal classifier 16070 is configured to classify the source ofthe acoustic wave detected and analyzed by the signal analyzer 16068.For example, the signal classifier 16070 can be, or can include, one ormore CNNs configured to use deep-learning techniques to determine, frominformation that the signal analyzer 16068 generates, the respectiveprobabilities that the source of the detected and analyzed acousticsignal belongs to classes (e.g., walking human, running human, multiplehumans, a moving vehicle) for which the one or more CNNs are trained.

Still referring to FIG. 16, alternate embodiments of the system 16016are contemplated. For example, embodiments described in conjunction withFIGS. 1-15 and 17-25 may be applicable to the system 16016 of FIG. 16.

Optical Detector Sub-System

The optical receiver 16024 described in conjunction with FIGS. 16, 17,and 18 is comprised of photodetector sub-systems. Any of the manywell-known photo-detecting techniques and devices may be employed.Possible implementation of the photodetection sub-systems will now bediscussed.

Refer to FIG. 17, like parts correspond to like numbers. Optical signalsenter the photodetector sub-system via optical paths 17001 and 17003,which are extensions of the paths 16034 and 16038 in the case ofreceiver 16024. The optical signals are equally split by optical coupleror beam splitter, 17005. The optical signal on path 17007 is compositesignal comprised of half the optical power of path 17001 and half of theoptical power arriving on path 17003. The optical signal on path 17007is illuminated on optical detector 17011. The photocurrent of opticaldetector 17011 flows into electrical conductor 17015. Likewise, theoptical signal on path 17009 is comprised of half the optical power onpath 17001 and half of the optical power on path 17003. The opticalsignal on path 17009 is illuminated on optical detector 17013. Thephoto-current of optical detector 17013 flows out of electricalconductor 17015. Therefore, the photo-currents of optical detectors17011 and 17013 are subtracted at electrical conductor or node 17015.

Photo-detectors 17011 and 17013 are, at least ideally, precisely matchedin responsivity. The differential photocurrent on electrical conductor17015 is input into pre-amplifier 17017, amplified and is passed toelectrical output 17019. The differential nature of the photo-detectionrejects either of the self-optical interference power of the signals onpaths 17001 and 17003 and receives only the cross-interference powerbetween the two optical signals on paths 17001 and 17003. Thisparticular optical detector architecture is called a balanced heterodyneoptical detection scheme. The scheme is 3 dB more sensitive than allother heterodyne optical detection methods and offers the distinctadvantage of rejecting local-oscillator noise.

FIG. 18 illustrates an alternative photo-detection scheme to FIG. 17.Light waves enter the receiver at paths 17001 and 17003. The opticalcoupler or beam splitter 17005 combines the light waves on paths 17001and 17003 into a composite light wave on path 17007. The composite lightwave on path 17007 illuminates the optical detector 17011. Thephoto-current of the optical detector caused by the self-interferenceand cross interference of light waves originating from optical paths17001 and 17003 passes through conductor 17015 a, is amplified bypre-amplifier 17017 and is passed to electrical output 17019.

The optical detector sub-system of FIGS. 17 and 18 correspond to opticalreceiver 16024 of FIG. 16. Paths 17001 and 17003 correspond to 16034 and16038 and output 17019 corresponds outputs 16036 from optical receiver16024. Either of the photo-detection schemes of FIG. 17 or 18 can beused for the optical receiver 16024. However, the photodetection schemeof FIG. 17 may be more suitable for some applications.

Still referring to FIGS. 17-18, alternate embodiments of the opticaldetector/receiver/modulator/mixer 16024 are contemplated. For example,embodiments described in conjunction with FIGS. 1-16 and 19-25 may beapplicable to the optical detector/receiver/modulator/mixer 16024 ofFIGS. 17-18.

Programmable Correlator System

Referring to FIG. 19, the composite radio frequency signal, or r.f.composite reference beat signal, which electronically represents thereceived time-delay multiplexed optical signal, or compositeback-propagation CW light wave E_(b)(t) (redirected optical signal), isinput into the correlator circuit 16030 at electrical input 16036. Thecomposite radio frequency signal is n-way split with power splitter19003 into a plurality (which upwardly may include a very large number,for instance 5,000) of electronic pathways including 19011, 19013 and19015. The master correlation code c(t) is input into the correlatorsystem 16030 at electrical input 19054. The correlation code isdistributed to such a plurality of programmable delay circuits including19021, 19023, and 19025. Each programmable delay circuit delays themaster correlation code by the delay suitable to decode/demultiplex eachtime-delay multiplexed channel. The plurality of programmable delaycircuits including 19021, 19023, and 19025 output a plurality of delayedcorrelation codes including those on electrical pathways 19031, 19033,and 19035, respectively. The corresponding plurality of delayedcorrelation codes including those on electrical pathways 19031, 19033,and 19035 are multiplied by a corresponding plurality of multipliers (orbalanced mixers) including 19041, 19043, and 19045, respectively, by theradio-frequency signal on the plurality of electronic pathways including19011, 19013, and 19015, which are amplified by a correspondingplurality of amplifiers including 19061, 19063, and 19065, respectively,to produce the corresponding plurality of outputs including O₁, O₂, . .. , O_(n) (on lines 16042, 16044, and 16046) respectively. Each of theoutputs therefore produces the corresponding demultiplexed signal whichis time-gated by the corresponding time-delay of the correlation code.The correlator system 16030 of FIG. 19 is an example implementation ofthe correlation system 16030 of FIG. 16.

The output O₁ corresponds to signal B(t, L₁), which is hereinbeforediscussed. The outputs O₁, O₂, . . . , O_(n) on lines 16042, 16044, and16046, respectively, correspond to signals S₁, S₂, . . . , S_(n), whichin turn are based upon the spatial delay associated with distances L₁,L₂, . . . , L_(n) indicated in FIG. 16. These spatial delays are basedon the time of propagation for fly-back travel along these distances,which are arbitrary and programmable. The time-delay multiplexing of theoptical signals comprising the composite back-propagating optical signal(redirected optical signal) on path 16038 of FIG. 16 arises from aplurality (which upwardly may include a very large number, for instance5,000) of spatial locations causing a like plurality of time-delays. Thecorrelator system 16030 spatially separates the components of the r.f.composite difference beat signal into channels, which each uniquelyrepresent an optical signal (a component of the redirected opticalsignal) at a single spatial location.

The correlator system 16030 allows the spatial sampling of the opticalsignals so that a virtual array can be formed along the fiber span 16002of FIG. 16.

Still referring to FIG. 19, alternate embodiments of the correlatorcircuit/system 16030 are contemplated. For example, embodimentsdescribed in conjunction with FIGS. 1-18 and 20-25 may be applicable tothe correlator circuit/system 16030 of FIG. 19.

Phase Demodulation System

The embodiment of phase demodulator circuit 16048 of FIG. 20, has twouses in system 16016. It either: (i) receives the outputs of the justdescribed r.f correlator subsection 16030, or (ii) is part of theintegration of the polarization fading reduction system of U.S. Pat. No.6,043,921 (as discussed above).

Referring to FIG. 20, the phase demodulation circuit 16048 is comprisedof a plurality (which upwardly may include a very large number, forinstance 5,000) of phase demodulators, 20081, 20083 and 20085. Theinputs to the plurality of phase demodulators, 16042, 16044, and 16046(the correlator outputs O₁, O₂, . . . , O_(n) discussed previously) arephase demodulated with phase demodulators 20081, 20083, and 20085,respectively. The outputs of these demodulators are passed on electricalpathways 16050, 16052, and 16054, respectively.

Referring to FIG. 21, an example block diagram of any one of the justdiscussed phase demodulators 20081, 20083, and 20085 is shown as part21300. The input electrical path 21301 corresponds to any one ofelectrical path 16042, 16044, 16046, etc. of the plurality of phasedemodulators. The output electrical path 21319 corresponds to any one ofelectrical path 16050, 16052, 16054, etc. of the plurality of phasedemodulators. A correlation-system output such as O₁, O₂, . . . , O_(n)is passed via electrical path 21301 into a bandpass filter 21303. Thebandpass filter 21303 passes only a band of radian frequencies in thevicinity of Δω so that only B(t, Lm) passes through the filter (where mis an integer corresponding to the particular channel). The band-passedsignal passes from 2103 via electrical path 21305 to amplitude control21307. Amplitude control 21307 is either an analog automatic gaincontrol circuit, an electronic clipper circuit, or a combinationthereof. The amplitude control 21307 removes amplitude variations due topolarization fading or other types of signal fading. Because the signalB(t, L_(m)) is a result of a heterodyne interference, the phase remainsthe same after clipping. It is to be appreciated that otherphase-demodulation schemes for fiber-optic signals use a phase-carriertechnique that does not allow the clipping operation. Clipping is asuitable amplitude-control mechanism. The amplitude control 21307 passesan amplitude-stabilized signal via electrical path 21309 to I& Qdemodulator 21311. The I& Q demodulator 21311 removes the carrier, thatis it shifts the center radian frequency of the amplitude-stabilizedB(t, L_(m)) from Δω down to zero. The I& Q demodulator 21311 outputs avoltage proportional to cos(Φ_(m)) on electrical path 21313 and avoltage proportional to sin(Φ_(m)) on electrical path 21315. Thecos(Φ_(m)) and sin(Φ_(m)) proportional voltages on electrical paths21313 and 21315, respectively, are converted into an output signalproportional to Φ_(m) on electrical path 21319 by the phase detector21317.

Reviewing the previous discussion, the plurality of phase demodulators20081, 20083, and 20085 of FIG. 20 each function like the block diagramof 21300 of FIG. 21. The plurality (which upwardly may include a verylarge number, for instance 5000) of phase demodulators 21300 convert toa like plurality of signals B(t, L₁), B(t, L₂), . . . , B(t, L_(n)) intoa like plurality of signals proportional to Φ₁, Φ₂, . . . , Φ_(n), whichcorrespond to optical signals S₁, S₂, . . . , S_(n).

Still referring to FIGS. 20-21, alternate embodiments of thephase-demodulation circuit 16048 and phase demodulator 21300 arecontemplated. For example, embodiments described in conjunction withFIGS. 1-19 and 22-25 may be applicable to the phase-demodulator circuit16048 of FIG. 20 and the phase demodulator 21300 of FIG. 21.

I & Q Demodulator

An example implementation of the I& Q demodulator 21311 of FIG. 21 willnow be presented. Referring to FIG. 22, an amplitude-stabilized B(t,L_(m)) signal (originating from the amplitude control 21307 of FIG. 21)is passed on electrical path 21309 to a power splitter 22403. Half ofthe signal power exiting from power splitter 22403 is passed to analogmixer, balanced mixer, Gilbert cell, or analog multiplier 21413 viaelectrical path 21411. The other half of signal power exiting from powersplitter 21403 is passed to analog mixer, balanced mixer, Gilbert cell,or analog multiplier 21423 via electrical path 21421.

The reference oscillator 21451 generates an electronic wave proportionalto cos(Δωt). In accordance with known principles of heterodyning lightwaves having fixed phase relationships, heterodyning the redirectedoptical signal from the fiber 16002 (FIG. 16) with the optical localoscillator signal from the acousto-optic modulator 16032 can produce adifference beat signal small enough to be in the r.f signal range, butwith the frequency difference sufficiently high to provide theheterodyning with a band pass allowing transforming a given binary coderate into corresponding code components of the beat signal, such as thecode rate of the PRN code sequence produced by PRN code generator 16022.This reference oscillator wave is passed from the reference oscillator22451 via the electrical path 22453 to amplifier 22455. The wave isamplified by amplifier 22455 and passed to hybrid coupler 22459 viaelectrical path 22447. The hybrid coupler splits the amplified referenceoscillator electronic wave into two components, one proportional tocos(Δωt) on electrical path 22417 (providing the “I”, or In-phasereference), and one proportional to sin(Δωt) on electrical path 22427(providing the “Q”, or Quadrature-phase reference).

The In-phase reference on electrical path 22417 is multiplied (orfrequency mixed) with the signal on electrical path 22411 by multiplier22413 to produce the output on electrical path 22415. The signal onelectrical path 22415 is amplified by amplifier 22431 and passed toelectronic lowpass filter 22435 via electrical path 22433. The lowpassfilter 22435 removes high-frequency components of the multiplication orfrequency-mixing process and results in an output at electrical path21313 that is proportional to cos(Φ_(m)).

The Quadrature-phase reference on electrical path 22427 is multiplied(or frequency mixed) with the signal on electrical path 22421 bymultiplier 22423 to produce the output on electrical path 22425. Thesignal on electrical path 22425 is amplified by amplifier 22441 andpassed to electronic lowpass filter 22445 via electrical path 22443. Thelowpass filter 22445 removes high-frequency components of themultiplication or frequency-mixing process and results in an output atelectrical path 21315 that is proportional to sin(Φ_(m)).

Still referring to FIG. 22, alternate embodiments of the I& Qdemodulator 21311 are contemplated. For example, embodiments describedin conjunction with FIGS. 1-21 and 23-25 may be applicable to the I& Qdemodulator 21311 of FIG. 22.

Phase Detector

Referring to FIG. 23, example implementations of the phase detector21317 of FIG. 21 is presented. An example digital phase detectorimplementation, 21317, is shown in the block diagram. The signalproportional to cos(Φ_(m)) on electrical path 21313 is converted to adigital code or number by analog-to-digital converter (hereafter, A/D)23513. The digital number proportional to cos(Φ_(m)) is input into thedigital signal processor 23501 via electrical path 23515. The signalproportional to sin(Φ_(m)) on electrical path 21315 is converted to adigital code or number by A/D 23523. The digital number proportional tosin(Φ_(m)) is input into the digital signal processor 23501 viaelectrical path 23525. The digital signal processor converts the numbersproportional to sin(Φ_(m)) and cos(Φ_(m)) into a number proportional toΦ_(m) as follows.

Suppose the constant of proportionality for the sin(Φ_(m)) andcos(Φ_(m)) is V_(m). Then the digital signal processor can optimallyselect estimates of Φ_(m) and V_(m) to minimize the calculated errorfunction:

∈({circumflex over (Φ)}_(m) ,{circumflex over (V)} _(m))=((V _(m)cos(Φ_(m))−{circumflex over (V)} _(m) cos({circumflex over(Φ)}_(m)))²+(V _(m) sin(Φ_(m))−{circumflex over (V)} _(m)sin(Φ_(m)))²)  (42)

The digital signal processor can also calculate Φ_(m) directly by takingthe inverse tangent function or the inverse cotangent function:

$\begin{matrix}{\Phi_{m} = {{a\;{\tan\left( \frac{V_{m}{\sin\left( \Phi_{m} \right)}}{V_{m}{\cos\left( \Phi_{m} \right)}} \right)}} = {{\alpha cot}\left( \frac{V_{m}{\cos\left( \Phi_{m} \right)}}{V_{m}{\sin\left( \Phi_{m} \right)}} \right)}}} & (43)\end{matrix}$

If desired, the digital signal processor can also implement thedifferentiate and cross multiply (hereafter DCM) algorithm. The DCMmethod is as follows. The digital representation of the signalsproportional to sin(Φ_(m)) and cos(Φ_(m)) are temporally differentiatedand cross multiplied by the non-differentiated signals. The resultU_(m)(t) is integrated to produce the desired output, Φ_(m).Mathematically, this algorithm is:

$\begin{matrix}{{{U_{m}(t)} = {{V_{m}{\sin\left( \Phi_{m} \right)}{\frac{\partial}{\partial_{t}}\left( {V_{m}{\cos\left( \Phi_{m} \right)}} \right)}} - {V_{m}{\cos\left( \Phi_{m} \right)}\frac{\partial}{\partial_{t}}\left( {V_{m}{\sin\left( \Phi_{m} \right)}} \right)}}}\mspace{79mu}{{U_{m}(t)} = {{V_{m}^{2}\left( {\left( {\cos\left( \Phi_{m} \right)} \right)^{2} + \left( {\sin\left( \Phi_{m} \right)} \right)^{2}} \right)}\frac{\partial\Phi_{m}}{\partial t}}}\mspace{79mu}{{U_{m}(t)} = {V_{m}^{2}\frac{\partial\Phi_{m}}{\partial t}}}\mspace{79mu}{\Phi_{m} = {\frac{1}{V_{m}^{2}}{\int{U_{m}{\partial j}}}}}} & (44)\end{matrix}$

The digital signal processor 23501 converts the signals arriving onelectrical paths 23515 and 23525 into a digital output proportional toΦ_(m) on electronic path 23503. Optionally, the digital output is passedon electronic path 23505 to some other data sink such as a computermemory. The digital signal proportional to Φ_(m) on electronic path23503 is converted back to an analog signal on electrical path 21319 bydigital-to-analog converter 23507. By way of a summarization, theexample digital phase detector 21317 accepts inputs 21313 and 21315,which originate from the I & Q demodulator 21311 of FIG. 21, and thedigital phase detector 21317 outputs the phase signal Φ_(m) onelectrical path 21319. Optionally, any of other well-knownimplementations of digital phase detectors may be employed.

Still referring to FIG. 23, alternate embodiments of the phase detector21317 are contemplated. For example, embodiments described inconjunction with FIGS. 1-22 and 24-25 may be applicable to the phasedetector 21317 of FIG. 23.

Refer to FIG. 24, an example analog phase detector implementation 21317′is shown in the block diagram. The example analog phase detector 21317′shown in FIG. 24 implements an analog version of the DCM algorithmdiscussed in the previous text. The signal proportional to cos(Φ_(m)) onelectrical path 21313 is input into analog temporal differentiator 24613and analog multiplier 24617. The signal proportional to sin(Φ_(m)) onelectrical path 21315 is input into analog temporal differentiator 24623and analog multiplier 24627. The differentiated cosine term on signalpath 24625 is multiplied by the sine term on electrical path 21315 byanalog multiplier 24627 producing the signal on electrical path 24629.The differentiated sine term on electrical path 24615 is multiplied bythe cosine term on electrical path 21313 by analog multiplier 24617producing the signal on electrical path 24619. The signals on electricalpaths 24619 and 24629 are applied as inputs to differential summer24631. The output of differential summer on electrical path 24633, whichis the result of the differentiated sine and cosine product beingsubtracted from the differentiated cosine and sine product, correspondsto U_(m)(t) of the DCM discussion. The signal on electrical path 24633is integrated by analog integrator 24635 to produce the analog phasedetector output proportional to Φ_(m) on electrical path and output21319. By way of summarization, the example analog phase detector 21317accepts inputs 21313 and 21315, which originate from the I & Qdemodulator 21311 of FIG. 21, then the analog phase detector outputs thephase signal Φ_(m) on electrical path 21319. Optionally, any of otherwell-known implementations of analog phase detectors may be employed.

Still referring to FIG. 24, alternate embodiments of the phase detector21317′ are contemplated. For example, embodiments described inconjunction with FIGS. 1-23 and 25 may be applicable to the phasedetector 21317′ of FIG. 24.

Programmable Phase Difference

The example programmable phase difference implementation shown as part16060 of FIG. 25 corresponds to phase differencer 16060 shown as a blockin FIG. 16. Referring to FIG. 25, the plurality (which upwardly mayinclude a very large number, for instance 5,000) of demodulated signalsproportional to optical signal phases Φ₁, Φ₂, . . . , Φ_(n) are inputinto the programmable phase signal switching and routing network 25701via electrical paths 16050, 16052, and 16054, respectively. Network25701 programmably selects on a basis of timed relation to codegenerator 16022 and routes on a basis of conventional “hold-in memory”and “transfer-from-memory”, a plurality (which upwardly may include avery large number, for instance 5,000) of pairs of phase signals onto aplurality (which upwardly may include a very large number, for instance5,000) of pairs of electronic paths 25711 and 25713, 25731 and 25733,and 25751 and 25753. The plurality of routed pairs of phase signals areapplied to the corresponding subtractors 25715, 25735, and 25755 asshown in FIG. 25. The plurality of phase pairs on electronic pairs ofpaths 25711 and 25713, 25731 and 25733, and 25751 and 25753 aresubtracted by subtractors 25715, 25735, and 25753, respectively, and thedifferential signals are output on a corresponding plurality ofelectrical paths 16062, 16064, and 16066, respectively. The followingdescription focuses on the differencing channel output on electricalpath 16062, it being understood that the modes of operation of otherdifferencing channels in network 25701 are the same. Programmable phaseswitching and routing network 25701 selects one of the phase signals onone of the electrical paths 16050, 16052, or 16054 and routes the signalto electrical path 25711. The signal on electrical path 25711 isselected to be proportional to Φ_(j) (where j is of the set 1, 2, . . ., n). Network 25701 also selects another of the phase signals on one ofthe other of the plurality of electronic paths 16050, 16052, or 16054and routes the signal to electrical path 25713. The signal of electricalpath 25713 is selected to be proportional to Φ_(k) (where k is of set 1,2, . . . , n). The signal on electrical path 25711 is subtracted fromthe signal on electrical path 25713 by subtractor 25715. The output ofsubtractor 25715 is passed on via electrical path 16062 and isproportional to ΔΦ_(kj) hereinabove discussed. Employing this mode,network 25701 programmably makes selection from optical signal phasesΦ₁, Φ₂, . . . , Φ_(n) to provide other differential phase outputs onelectrical paths 16062, 16064, and 16066. This may include a very largenumber of differential phase signals, for instance 5000. As analternative to the just described type of circuitry employingsubtractors 25715, 25735, and 25755 any of other well-known forms ofproducing a differential signal may be employed.

Still referring to FIG. 25, alternate embodiments of the phasedifferencer 16060 are contemplated. For example, embodiments describedin conjunction with FIGS. 1-24 may be applicable to the phasedifferencer 16060 of FIG. 25.

An Alternative Viewpoint of the Partitioning of System 2.

As an alternative to the viewpoint inferable from the preceding sequencediscussing FIG. 16, system 16016 may be considered as partitioned into:(i) an optical network for illuminating an optical fiber sensing span,or other light propagation medium sensing span, and retrieving backpropagating portions of the illumination; and (ii) a photoelectronicnetwork for establishing virtual sensors at predetermined locationsalong the span and picking up external physical signals incident to, orimpinging upon, the sensors.

In general, the optical network for the illumination of, and for theretrieval of back-propagation from, fiber span 16002 comprisestransmitter laser 16004, directional optical coupler 16010, and opticalfiber, or other light propagation medium, 16002.

The photoelectronic network for establishing virtual sensors and pickingup signals therefrom generally comprises two subdivisions. Onesubdivision provides a cyclically reiterative autocorrelatable form ofmodulation of the light wave illuminating fiber span 16002. Thismodulation is in the form of reiterated sequences havingautocorrelatable properties. The other subdivision takes the retrievedback propagation and performs a heterodyning therewith to obtain an r.f.beat signal. It then picks up the signal from the virtual sensors byautocorrelation and further processes it into more useful forms.

In general, the subdivision providing the cyclical reiterativemodulation of sequences illuminating fiber span 16002 comprises mastercorrelation code generator 16022 (via one of its electrical pathwayoutputs) and optical modulator 16024.

In general, the subdivision for performing heterodyning with and pickingup of virtual sensor signals from the retrieved back propagation(redirected optical signal) from fiber span 16002 includes a sequence ofelements that perform processing upon the retrieved back propagation.First in the sequence of processing elements is an optical receiver16024, which photo detects interference power “derived” by heterodyningthe back-propagated illumination portion retrieved from fiber span 16002with the output of an effective local oscillator (acousto-opticmodulator) 16032. Laser 16004 and the output of the modulator 16032 areoperated with a frequency difference to produce an r.f. beat signal, Δω.Then correlation system 16030 receives as one of its inputs anotherelectrical pathway output from master correlation code generator 16022,and provides a series of channels which in turn respectively providepredetermined time delays in relation to the timing base of cyclicreiterative code generator 16022, to perform a series ofautocorrelations of the respectively delayed inputs from code generator16022 with the signal Δω. This picks up r.f. signals respectivelyrepresentative of the affects in the light-wave domain of the externalphysical signals incident upon the respective virtual sensor. Phasedemodulator system 16048 provides a linear phase signal derived fromsuch r.f. signals representative of optical signals at the respectivevirtual sensors. Programmable phase differencer 16060 processes pairs ofthese linear phase signals occurring across segments of fiber span 16002between programmably selected pairs of the virtual sensors.

Following is another overview description which more particularly callsattention to an aspect of one or more embodiments that the systemelements which perform the autocorrelation enable providing an output inthe form of an r.f. counterpart of a light-wave time-domainreflectometry output of signals incident to the virtual sensors aslight-wave time-domain reflectometry outputs a CW light wave modulatedby a continuously reiterated binary pseudorandom code sequence islaunched into an end of a span of ordinary optical fiber cable. Portionsof the launched light wave back propagate to the launch end from acontinuum of locations along the span because of innate fiber propertiesincluding Rayleigh scattering. This is picked off the launch end andheterodyned to produce an r.f. beat signal. The r.f. beat signal isprocessed by a plurality (which can be thousands) of correlator typebinary pseudonoise code sequence demodulators respectively operated indifferent delay time relationships to the timing base of the reiteratedmodulation sequences. The outputs of the demodulators provide r.f.time-domain reflectometry outputs representative of signals (e.g.,acoustic pressure waves) incident to virtual sensors along the fiber atpositions corresponding to the various time delay relationships.

Following is still another overview description which more particularlycalls attention to an aspect of embodiments that the system elementsperforming the autocorrelation enable detection of unique spectralcomponents representing a phase variations of external signals incidentto the virtual sensors. A CW light wave modulated by a continuouslyreiterated pseudorandom code sequence is launched into an end of a spanof ordinary optical fiber cable. Portions of the launched light waveback propagate to the launch end from a continuum of locations along thespan because of innate fiber properties including Rayleigh scattering.This is picked off the launch end and heterodyned producing an r.f. beatsignal. The r.f. beat signal is processed by a plurality (which can bethousands) of correlator type pseudonoise code sequence demodulation andphase demodulator units, operated in different time delay relationshipsto the timing base of the reiterated modulation sequences. These unitsprovide outputs representative of phase variations in respective uniquespectral components in the r.f. beat signal caused by acoustic, or otherforms of signals, incident to virtual sensors at fiber positionscorresponding to the various time delay relationships.

Following is yet another overview description which more particularlycalls attention to an aspect of embodiments that a pair of the differentdelay time relationships of the autocorrelation system elements areeffective to establish a virtual increment of the optical fiber span,and that a subtractor circuit of phase differencer 16060 enablesrepresenting the differential phase signal across the virtual increment.A CW light wave modulated by a continuously reiterated pseudorandom (PN)code sequence is launched into an end of a span of ordinary opticalfiber cable. Portions of the launched light wave back propagate to thelaunch end from a continuum of locations along the span because ofinnate fiber properties including Rayleigh scattering. This is pickedoff the launch end and heterodyned producing an r.f. beat signal. Ther.f. beat signal is processed by a plurality (which can be thousands) ofcorrelator pseudonoise code sequence demodulation and phase demodulatorunits operated in different delay time relationships to the timing baseof the reiterated modulation sequences. Pairs of outputs of the unitsare connected to respective subtractor circuits, each providing a signalrepresentative of signal differential of incident acoustic signals, orother forms of signals, across virtual increments of the spanestablished by a pair of said delay time relationships.

Mandrel-Modified Embodiment

FIG. 2 illustrates a so-called fiber-on-an-airbacked mandrel assembly1006, useful in applications in which a fiber optic span 1010 is to beimmersed in a liquid medium. Assembly 1006 comprises a hollowcylindrical mandrel form 2000 having formed therein a sealed centralchamber 2004 containing air or other gaseous medium, which iscompressible relative to the liquid medium. A segment of span 1010 of aROSE system 16016 (FIG. 16), is helically wound about the cylindricalexterior surface of form 2000, and suitably fixedly bonded to thesurface. The cylindrical wall of form 2000 is of a material so-chosenand of a thickness so chosen to form a containic membrane with a hoopstiffness that enables acoustic pressure wave signals incident uponassembly 1006 to be transformed into mandrel radial dimensionalvariations. As a result of mandrel 1006's geometry, these radialvariations result in magnified longitudinal strain variations in fiber1010. It is to be appreciated that the physical structure of assembly1006 inherently provides a spatial succession of two locations along thefiber span, which a phase signal switch and routing network could selectand route to become the virtual bounding positions of a differentialphase signal virtual sensor. This is to say, positioning a mandrel woundspan 1010 as a segment of a system total span 16002 of ROSE system 16016can facilitate providing a sequential pair of virtual sensor locationsalong a span 16002, and the provision of a corresponding pair of delaycircuits in correlator circuit 16030 would cause mandrel 1006 to operateas a differential signal sensor.

Advantages and New Features

Embodiments enable the interrogation or time-delay correlationalmultiplexing and demultiplexing of optical phase signals.

Embodiments enable the interrogation of ROSE (Rayleigh OpticalScattering and Encoding) fiber optic sensors. Embodiments enable thespatial sorting and separation of the temporal optical phases ofbackscattered optical signals arising from a plurality (which upwardlymay include a very large number, for instance 5,000) of virtual opticalsensors along fibers or other optical mediums. Embodiments enable thespatial decoding of backscattered optical signals with a bandwidth oftens of kilohertz. Embodiments enable the sensor locations along thefiber to be programmable. Embodiments allow the electronic separation orsegmentation of the array of fiber sensors into programmable boundedlengths and positions. Because the correlation signal c(t) can bedesigned to be a continuous wave, embodiments increase the averageoptical power considerably over conventional pulsed optical phase sensorinterrogation methods. Because the correlation signal c(t) can be chosento have spectrum spreading properties for which dispreading electroniccircuitry is readily available, undesired optical fiber system noises,such as reflection discontinuity noises due to cable couplings, can bematerially attenuated.

In hypothetically assessing the potential achievable by embodiments withregard to employment of a common grade of optical fiber cable buriedbeneath the ground surface as a perimeter intrusion monitoring fiberspan, the following assumptions have been made: (i) signal to noiseratio (S/N) degradation of Rayleigh effect light propagation in such anoptical fiber cable are assumed to be 0.5 db/km; (ii) it is assumedthere is a requirement for bandwidth of ten times that of thegeo-acoustic intruder signal needs to be detected; (iii) and digitalcircuitry functions are performed employing conventional “high end”clock rates. Using these assumptions and employing conventionalsingle-mode or multimode fiber buried 6-12 inches underground, and usingconventional engineering methodology for noise-effect prediction, it canbe shown that ROSE system 16016 has the potential of sensing intrudercaused geoacoustic, (i.e., seismic) signals along a length of fiber spanline as long as 8 km or 5 miles. (This assessment is based upon S/Ndegradations for fly-back travel of signals from the interrogationlaunch end of fiber span 16002 to its remote end and back.) Thehypothetical segment resolution capability with such an 8-km, or 5-mileline, would be 1 meter.

Embodiments provide a new capability of heterodyne optical phasedetection without resorting to dithered phase carrier methods. The phasedemodulation method introduces heterodyne I& Q demodulation to producecosine and sine phase components, clipped signal amplitude stabilizationtechniques and digital-signal-processing-based phase detection. Thespatially differential phase detection method provided by embodimentsenables the rejection of unwanted lead-in fiber phase signals.

The details, materials step of operation and arrangement of parts hereinhave been described and illustrated in order to explain the nature ofthe embodiments and techniques disclosed herein. Many modifications inthese are possible by those skilled in the art within the teachingsherein. For example, while in system 16016 the transformation fromoptical to r.f. signal takes place prior to processing by programmablecorrelator 16030, it is within the skill of the art to design opticalreceiver 16024 and correlator system 16030 to have the transformationtake place otherwise. Also, the acoustic-optic modulator 16032 can be aBragg Cell. The diffracted optical wave exiting the acousto-opticmodulator will be Doppler shifted by an impinging-driving RF wave, thatis translated into a sound wave in the acousto-optic modulator, and theso-called Bragg shifted-diffracted optical wave will exit theacousto-optical modulator with an optical frequency equivalent to aphase-locked laser if two lasers were used in a phase-locked-loopconfiguration where the second laser generates the opticallocal-oscillator signal. As described above, the acousto-opticallygenerated light wave is sent along optical pathway 16034 and becomes thelocal oscillator input to heterodyne photoreceiver 16024. Accordingly,it is to be understood that changes may be made by those skilled in theart within the principle and scope of the teachings herein.

The methods and techniques described herein may be implemented in analogelectronic circuitry, digital electronic circuitry, or with aprogrammable processor (for example, a special-purpose processor, ageneral-purpose processor such as a computer, a microprocessor, ormicrocontroller) or other circuit (for example, an FPGA), firmware,software, or in combinations of them. Apparatuses embodying thesetechniques may include appropriate input and output devices, aprogrammable processor, and a storage medium tangibly embodying programinstructions for execution by the programmable processor. A processembodying these techniques may be performed by a programmable processorexecuting a program of instructions to perform desired functions byoperating on input data and generating appropriate output. Thetechniques may advantageously be implemented in one or more programsthat are executable on a programmable system including at least oneprogrammable processor coupled to receive data and instructions from,and to transmit data and instructions to, a data storage system, atleast one input device, and at least one output device. Generally, aprocessor will receive instructions and data from a read-only memoryand/or a random-access memory. Storage devices suitable for tangiblyembodying computer program instructions and data include all forms ofnon-volatile memory, including by way of example semiconductor memorydevices, such as EPROM, EEPROM, and flash memory devices; magnetic diskssuch as internal hard disks and removable disks; magneto-optical disks;and DVD disks. Any of the foregoing may be supplemented by, orincorporated in, specially designed application-specific integratedcircuits (ASICs).

A number of embodiments of the invention defined by the following claimshave been described. Nevertheless, it will be understood that variousmodifications to the described embodiments may be made without departingfrom the spirit and scope of the claimed invention. Accordingly, otherembodiments are within the scope of the following claims.

Example Embodiments

Example 1 includes a mandrel, comprising: a connector having first andsecond aligned openings and a third opening between the first and secondopenings; and a form coupled to the connector at the third opening,having an outer surface, a cavity, an end, and fourth and fifth openingsbetween the cavity and the outer surface, and configured to receive anoptical fiber that extends into the first opening of the connector,through the third opening of the connector, into the cavity at the endof the form, and through the fourth opening of the form, that forms oneor more turns around the outer surface of the form, and that extendsthrough the fifth opening of the form into the cavity, out from thecavity at the end, into the third opening of the connector, and out fromthe second opening of the connector.

Example 2 includes the mandrel of Example 1, wherein the connectorfurther comprises: a first section having first and second ends at whichthe first and second openings are disposed; and a second section havinga first end coupled to the first section between the first and secondends of the first section, and having a second end at which the thirdopening is disposed.

Example 3 includes the mandrel of any of Examples 1-2, wherein theconnector is T shaped.

Example 4 includes the mandrel of any of Examples 1-3, wherein at leasta portion of the second section is orthogonal to the first section.

Example 5 includes the mandrel of any of Examples 1-4, wherein thesecond section comprises: a first portion that includes the first end ofthe second section and that is orthogonal to the first section; and asecond portion that includes the second end of the second section andthat is orthogonal to the first portion.

Example 6 includes the mandrel of any of Examples 1-5, wherein thesecond portion is moveable relative to the first portion.

Example 7 includes the mandrel of any of Examples 1-6, wherein thesecond portion is rotatable relative to the first portion.

Example 8 includes the mandrel of any of Examples 1-7, furthercomprising the optical fiber.

Example 9 includes the mandrel of any of Examples 1-8, furthercomprising a cover disposed over the optical fiber and the form.

Example 10 includes the mandrel of any of Examples 1-9, furthercomprising: the optical fiber; and at least one optical-signalredirector disposed in a portion of the optical fiber that forms one ormore turns around the outer surface of the form.

Example 11 includes the mandrel of any of Examples 1-10, furthercomprising shrink wrap disposed over the optical fiber and the form.

Example 12 includes a method, comprising: inserting an end of an opticalfiber through a first opening of a connector; inserting the end of thefiber through a second opening of the connector and into an end of acavity within a form; running the end of the fiber from the cavitythrough a first opening in the form; wrapping the fiber around an outerside of the form; running the end of the fiber back into the cavitythrough a second opening in the form; running the end of the fiber outfrom the end of the cavity; inserting the end of the fiber into thesecond opening of the connector; and running the end of the fiber outfrom the connector though a third opening of the connector.

Example 13 includes the method of Example 12, further comprisingattaching the form to the connector such that the end of the cavity isaligned with the second opening of the connector.

Example 14 includes the method of any of Examples 12013, furthercomprising moving the attached form relative to a portion of theconnector.

Example 15 includes the method of any of Examples 12-14, furthercomprising forming a cover over the fiber and the outer side of theform.

Example 16 includes the method of any of Examples 12-15, furthercomprising covering the fiber and the outer side of the form with shrinkwrap.

Example 17 includes a mandrel, comprising: an outer conduit having firstand second ends; an inner conduit disposed inside of the outer conduitand having first and second ends; a first end cap having an outer end,an inner end coupled to the first end of the inner conduit and having aperimeter, and an optical-fiber path extending between the outer end andthe perimeter; and a second end cap having an outer end, an inner endcoupled to the second end of the inner conduit and having a perimeter,and an optical-fiber path extending between the outer end and theperimeter.

Example 18 includes the mandrel of Example 17, wherein: the inner end ofthe first end cap includes an outer receptacle; the inner end of thesecond end cap includes an outer receptacle; and the first and secondends of the outer conduit are respectively disposed in the outerreceptacles of the first and second end caps.

Example 19 includes the mandrel of any of Examples 17-18, wherein theouter receptacles of the first and second end caps comprise respectiveslots.

Example 20 includes the mandrel of any of Examples 17-19, wherein: theinner end of the first end cap includes an inner receptacle; the innerend of the second end cap includes an inner receptacle; and the firstand second ends of the inner conduit are respectively disposed in theinner receptacles of the first and second end caps.

Example 21 includes the mandrel of any of Examples 17-20, wherein: theinner receptacles of the first and second end caps comprise respectivethreads; and the first and second ends of the inner conduit compriserespective threads that are engaged with the threads of the innerreceptacles of the first and second end caps, respectively.

Example 22 includes the mandrel of any of Examples 17-21, furthercomprising: a first fiber grip coupled to the outer end of the firstcap; and a second fiber grip coupled to the outer end of the second endcap.

Example 23 includes the mandrel of any of Examples 17-22, wherein: theouter end of the first end cap includes a receptacle; the outer end ofthe second end cap includes a receptacle; and the first and second fibergrips are respectively disposed in the receptacles of the outer ends ofthe first and second end caps.

Example 24 includes the mandrel of any of Examples 17-23, wherein: thereceptacles of the outer ends of the first and second end caps compriserespective threads; and the first and second fiber grips compriserespective threaded ends that are engaged with the threads of thereceptacles of the outer ends of the first and second end caps,respectively.

Example 25 includes the mandrel of any of Examples 17-24, wherein theinner and outer conduits are cylindrical.

Example 26 includes the mandrel of any of Examples 17-25, furthercomprising an optical fiber that extends from the outer end of the firstend cap, through the optical-fiber path of the first end cap, around theouter conduit one or more turns, through the optical-fiber path of thesecond end cap, and to the outer end of the second end cap.

Example 27 includes the mandrel of any of Examples 17-26, furthercomprising at least one optical-signal reflector disposed in a portionof the optical fiber that extends around the outer conduit one or moreturns.

Example 28 includes the mandrel of any of Examples 17-27, furthercomprising an optical fiber that extends through the first fiber grip,the outer end of the first end cap, and the optical-fiber path of thefirst end cap, around the outer conduit one or more turns, and throughthe optical-fiber path of the second end cap, the outer end of thesecond end cap, and the second fiber grip.

Example 29 includes the mandrel of any of Examples 17-28, furthercomprising: an optical fiber that extends from the outer end of thefirst end cap, through the optical-fiber path of the first end cap,around the outer conduit one or more turns, through the optical-fiberpath of the second end cap, and to the outer end of the second end cap;and an optical-fiber support member that extends from the outer end ofthe first end cap, through the inner conduit, and to the outer end ofthe second end cap.

Example 30 includes the mandrel of any of Examples 17-29, furthercomprising: an optical fiber that extends through the first fiber grip,the outer end of the first end cap, and the optical-fiber path of thefirst end cap, around the outer conduit one or more turns, and throughthe optical-fiber path of the second end cap, the outer end of thesecond end cap, and the second fiber grip; and an optical-fiber supportmember that extends through the first fiber grip, the outer end of thefirst end cap, the inner conduit, the outer end of the second end cap,and the second fiber grip.

Example 31 includes a method, comprising: inserting an optical fiberthrough an outer end of a first end cap and an optical-fiber path of thefirst end cap, and out from an outer perimeter of an inner end of thefirst end cap; wrapping the optical fiber one or more turns around anouter conduit having ends respectively engaged with the inner end of thefirst end cap and with an inner end of a second end cap; and insertingthe optical fiber through an optical-fiber path of the second end capfrom an outer perimeter of the inner end of the second end cap to anouter end of the second end cap.

Example 32 includes the method of Example 31, further comprising:inserting the optical fiber through a first fiber grip before insertingthe optical fiber through the outer end of the first end cap; andinserting the optical fiber through a second fiber grip after insertingthe optical fiber through the optical-fiber path of the second end cap.

Example 33 includes the method of any of Examples 31-32, furthercomprising: inserting the optical fiber through a first fiber gripbefore inserting the optical fiber through the outer end of the firstend cap; inserting the optical fiber through a second fiber grip afterinserting the optical fiber through the optical-fiber path of the secondend cap; attaching the first fiber grip to the outer end of the firstcap; and attaching the second fiber grip to the outer end of the secondend path.

Example 34 includes the method of any of Examples 31-33, furthercomprising: inserting the optical fiber through a first fiber gripengaged with the outer end of the first end cap before inserting theoptical fiber through the outer end of the first end cap; and insertingthe optical fiber through a second fiber grip engaged with the outer endof the second end cap after inserting the optical fiber through theoptical-fiber path of the second end cap.

Example 35 includes the method of any of Examples 31-34, furthercomprising inserting a fiber-support member through the first end cap,the outer conduit, and the second end cap.

Example 36 includes the method of any of Examples 31-35, furthercomprising: disengaging a fiber-support member from the optical fiberbefore inserting the optical fiber through the outer end of the firstend cap; inserting the fiber-support member through the first end cap,the outer conduit, and the second end cap; and reengaging thefiber-support member with the optical fiber after inserting the opticalfiber through the outer end of the second end cap.

Example 37 includes the method of any of Examples 31-36, furthercomprising inserting a fiber-support member through the first end cap,through an inner conduit that is disposed within the outer conduit andthat has first and second ends respectively engaged with the inner endsof the first and second end caps, and through the second end cap.

Example 38 includes the method of any of Examples 31-37, furthercomprising: disengaging a fiber-support member from the optical fiberbefore inserting the optical fiber through the outer end of the firstend cap; inserting the fiber-support member through the first end cap,through an inner conduit that is disposed within the outer conduit andthat has first and second ends respectively engaged with the inner endsof the first and second end caps, and through the second end cap; andreengaging the fiber-support member with the optical fiber afterinserting the optical fiber through the outer end of the second end cap.

Example 39 includes the method of any of Examples 31-38, furthercomprising: inserting the optical fiber through a first fiber gripbefore inserting the optical fiber through the outer end of the firstend cap; inserting the optical fiber through a second fiber grip afterinserting the optical fiber through the optical-fiber path of the secondend cap; disengaging a fiber-support member from the optical fiberbefore inserting the optical fiber through the outer end of the firstend cap; inserting the fiber-support member through the first fibergrip, through the first end cap, through an inner conduit that isdisposed within the outer conduit and that has first and second endsrespectively engaged with the inner ends of the first and second endcaps, through the second end cap, and through the second fiber grip; andreengaging the fiber-support member with the optical fiber afterinserting the optical fiber through the second fiber grip.

Example 40 includes an assembly, comprising: an optical fiber; and atleast one mandrel each including a respective outer conduit having firstand second ends and about which a respective portion of the opticalfiber is wound; a respective inner conduit disposed inside of the outerconduit and having first and second ends; a respective first end caphaving an outer end, an inner end coupled to the first end of the innerconduit and having a perimeter, and an optical-fiber path extendingbetween the outer end and the perimeter and within which a respectiveportion of the optical fiber is disposed; and a respective second endcap having an outer end, an inner end coupled to the second end of theinner conduit and having a perimeter, and an optical-fiber pathextending between the outer end and the perimeter and within which arespective portion of the optical fiber is disposed.

Example 41 includes a system, comprising: an optical fiber having anend; at least one mandrel each including a respective outer conduithaving first and second ends and about which a respective portion of theoptical fiber is wound; a respective inner conduit disposed inside ofthe outer conduit and having first and second ends; a respective firstend cap having an outer end, an inner end coupled to the first end ofthe inner conduit and having a perimeter, and an optical-fiber pathextending between the outer end and the perimeter and within which arespective portion of the optical fiber is disposed; and a respectivesecond end cap having an outer end, an inner end coupled to the secondend of the inner conduit and having a perimeter, and an optical-fiberpath extending between the outer end and the perimeter and within whicha respective portion of the optical fiber is disposed; and a signaldetector configured to direct a source optical beam into the end of theoptical fiber, to receive a redirected optical beam from the end of theoptical fiber, and to detect an acoustic signal incident on at least oneof the at least one mandrel in response to the redirected optical beam.

Example 42 includes the system of Examples 41, wherein each of the atleast one mandrel is disposed inline relative to the optical fiber.

Example 43 includes the system of any of Examples 41-42, wherein thesignal detector is configured to determine an amplitude and a frequencyof the detected acoustic signal in response to the redirected opticalbeam.

Example 44 includes the system of any of Examples 41-43, wherein thesignal detector is configured to determine a location of a source of thedetected acoustic signal in response to components of the redirectedoptical beam, each component corresponding to a respective one of the atleast one mandrel.

Example 45 includes the system of any of Examples 41-44, wherein thesignal detector is configured to detect an acoustic signal incident onat least one portion of the optical fiber that is external to the atleast one mandrel in response to the redirected optical beam.

Example 46 includes the system of any of Examples 41-45, wherein: thesignal detector is configured to detect an acoustic signal incident onat least one portion of the optical fiber that is external to the atleast one mandrel in response to the redirected optical beam; and thesignal detector is configured to determine a location of a source of thedetected acoustic signal in response to components of the redirectedoptical beam, each component corresponding to either a respective one ofthe at least one mandrel or to a respective one of the at least oneportion of the optical fiber.

Example 47 includes the system of an of Examples 41-46, furthercomprising a fiber-support member that extends through the respectivefirst end cap, the respective inner conduit, and the respective secondend cap of at least one of the at least one mandrel and that engages atleast one portion of the optical fiber external to the at least onemandrel.

Example 48 includes a method, comprising: sourcing an optical beam intoan end of an optical fiber that extends through an optical-fiber path ofa respective first end cap of each at least one mandrel, that is woundabout a respective outer conduit of each of the at least one mandrel,and that extends through an output-fiber path of a respective second endcap of each of the at least one mandrel fiber; receiving a redirectedoptical beam from the end of the optical fiber; and detecting anacoustic signal incident on at least one of the at least one mandrel inresponse to the redirected optical beam.

Example 49 includes the method of Example 48, further comprisingdetermining an amplitude and a frequency of the detected acoustic signalin response to the redirected optical beam.

Example 50 includes the method of any of Examples 48-49, furthercomprising determining a location of a source of the detected acousticsignal in response to components of the redirected optical beam, eachcomponent corresponding to a respective one of the at least one mandrel.

Example 51 includes the method system of any of Examples 48-50, furthercomprising detecting an acoustic signal incident on at least one portionof the optical fiber that is external to the at least one mandrel inresponse to the redirected optical beam.

Example 52 includes the method system of any of Examples 48-51, furthercomprising: detecting an acoustic signal incident on at least oneportion of the optical fiber that is external to the at least onemandrel in response to the redirected optical beam; and determining alocation of a source of the detected acoustic signal in response tocomponents of the redirected optical beam, each component correspondingto either a respective one of the at least one mandrel or to arespective one of the at least one portion of the optical fiber.

Example 53 includes a system, comprising: a light source configured togenerate a source optical signal; an optical assembly configured todirect the source optical signal into an end of an optical-fiberassembly that includes an optical fiber having at least one section eachwrapped multiple turns around a respective one of at least one mandreland each including respective mandrel zones, and to receive, from theend of the optical-fiber assembly, a return optical signal; and anelectronic circuit configured to select a first mandrel zone of one ofthe at least one section of the optical fiber in response to a firstcomponent of the return optical signal redirected by the first mandrelzone, and to detect an acoustic signal incident on the one of the atleast one mandrel around which the one of the at least one section ofthe optical fiber is wound in response to the first component of thereturn optical signal.

Example 54 includes the system of Examples 53, wherein the electroniccircuit is further configured to select the first mandrel zone inresponse to the first component of the return optical signal having acharacteristic that is closer to a target than at least one othercomponent of the return optical signal each redirected by a respectiveother mandrel zone of the one of the at least one section of the opticalfiber.

Example 55 includes the system of any of Examples 53-54, wherein theelectronic circuit is further configured to select the first mandrelzone in response to the first component of the return optical signalhaving a lower noise component than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber.

Example 56 includes the system of any of Examples 53-55, wherein theelectronic circuit is further configured to select the first mandrelzone in response to the first component of the return optical signalhaving a higher optical power than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber.

Example 57 includes the system of any of Examples 53-56, wherein theelectronic circuit is further configured to determine a frequency of theacoustic signal in response to the first component of the return opticalsignal.

Example 58 includes the system of any of Examples 53-57, wherein theelectronic circuit is further configured to determine a strength of theacoustic signal in response to the first component of the return opticalsignal.

Example 59 includes the system of any of Examples 53-58, wherein theelectronic circuit is further configured to determine an amplitude ofthe acoustic signal in response to the first component of the returnoptical signal.

Example 60 includes the system of any of Examples 53-59, wherein theelectronic circuit is further configured to determine a power of theacoustic signal in response to the first component of the return opticalsignal.

Example 61 includes the system of any of Examples 53-60, wherein theelectronic circuit is further configured: to select a second mandrelzone of the one of the at least one section of the optical fiber inresponse to a second component of the return optical signal redirectedby the second mandrel zone, and to detect the acoustic signal inresponse to the second component of the return optical signal.

Example 62 includes the system of any of Examples 53-61, wherein: thefirst mandrel zone is located along a half of the one of the at leastone mandrel in a lengthwise dimension; and the electronic circuit isfurther configured: to select a second mandrel zone of the one of the atleast one section of the optical fiber in response to a second componentof the return optical signal redirected by the second mandrel zone, thesecond mandrel zone is located along another half of the one of the atleast one mandrel in the lengthwise dimension, and to determine that theacoustic signal is incident on the one of the at least one mandrel inresponse to the first and second components of the return opticalsignal.

Example 63 includes the system of any of Examples 53-62, wherein: thefirst mandrel zone is located along a half of the one of the at leastone mandrel in a lengthwise dimension; and the electronic circuit isfurther configured: to select a second mandrel zone of the one of the atleast one section of the optical fiber in response to a second componentof the return optical signal redirected by the second mandrel zone, thesecond mandrel zone is located along another half of the one of the atleast one mandrel in the lengthwise dimension, to determine respectivefirst and second values of a characteristic of the first and secondcomponents of the return optical signal, and to determine that theacoustic signal is incident on the one of the at least one mandrel inresponse to a combination of the first and second values.

Example 64 includes the system of any of Examples 53-63, wherein: thefirst mandrel zone is located along a half of the one of the at leastone mandrel in a lengthwise dimension; and the electronic circuit isfurther configured: to select a second mandrel zone of the one of the atleast one section of the optical fiber in response to a second componentof the return optical signal redirected by the second mandrel zone, thesecond mandrel zone is located along another half of the one of the atleast one mandrel in the lengthwise dimension, to determine respectivefirst and second phases of the first and second components of the returnoptical signal, to determine that the acoustic signal is incident on theone of the at least one mandrel in response to a difference between thefirst and second phases.

Example 65 includes the system of any of Examples 53-64, wherein: thefirst mandrel zone is located along a half of the one of the at leastone mandrel in a lengthwise dimension; and the electronic circuit isfurther configured: to select the first mandrel zone in response to thefirst component of the return optical signal having a lower noisecomponent than at least one other component of the return optical signaleach redirected by a respective other mandrel zone of the one of the atleast one section of the optical fiber located along the half of the oneof the at least one mandrel, to select a second mandrel zone of the oneof the at least one section of the optical fiber located along anotherhalf of the one of the at least one mandrel in the lengthwise dimensionin response to a second component of the return optical signalredirected by the second mandrel zone having a lower noise componentthan at least one other component of the return optical signal eachredirected by a respective other mandrel zone of the one of the at leastone section of the optical fiber located along the other half of the oneof the at least one mandrel, to determine respective first and secondphases of the first and second components of the return optical signal,and to determine that the acoustic signal is incident on the one of theat least one mandrel in response to a difference between the first andsecond phases.

Example 66 includes the system of any of Examples 53-65, wherein: thefirst mandrel zone is located along a half of the one of the at leastone mandrel in a lengthwise dimension; and the electronic circuit isfurther configured: to select the first mandrel zone in response to thefirst component of the return optical signal having a higher opticalpower than at least one other component of the return optical signaleach redirected by a respective other mandrel zone of the one of the atleast one section of the optical fiber located along the half of the oneof the at least one mandrel, to select a second mandrel zone of the oneof the at least one section of the optical fiber located along anotherhalf of the one of the at least one mandrel in the lengthwise dimensionin response to a second component of the return optical signalredirected by the second mandrel zone having a higher optical power thanat least one other component of the return optical signal eachredirected by a respective other mandrel zone of the one of the at leastone section of the optical fiber located along the other half of the oneof the at least one mandrel, to determine respective first and secondphases of the first and second components of the return optical signal,and to determine that the acoustic signal is incident on the one of theat least one mandrel in response to a difference between the first andsecond phases.

Example 67 includes the system of any of Examples 53-66, wherein: thefirst mandrel zone is located along an end third of the one of the atleast one mandrel in a lengthwise dimension; and the electronic circuitis further configured: to select the first mandrel zone and at least onesecond mandrel zone located along the end third of the one of the atleast one mandrel in response to the first component of the returnoptical signal and at least one second component of the return opticalsignal respectively redirected by the at least one second mandrel zonehaving lower noise components than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the end third of the one of the at least one mandrel, to selectthird mandrel zones of the one of the at least one section of theoptical fiber located along another end third of the one of the at leastone mandrel in the lengthwise dimension in response to third componentsof the return optical signal redirected by the third mandrel zoneshaving lower noise components than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the other end third of the one of the at least one mandrel, todetermine respective phases of the first, second, and third phases ofthe first, second, and third components of the return optical signal,and to determine that the acoustic signal is incident on the one of theat least one mandrel in response to a combination of the first, second,and third phases.

Example 68 includes the system of any of Examples 53-67, wherein: thefirst mandrel zone is located along an end third of the one of the atleast one mandrel in a lengthwise dimension; and the electronic circuitis further configured: to select the first mandrel zone and at least onesecond mandrel zone located along the end third of the one of the atleast one mandrel in response to the first component of the returnoptical signal and at least one second component of the return opticalsignal respectively redirected by the at least one second mandrel zonehaving lower noise components than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the end third of the one of the at least one mandrel, to selectthird mandrel zones of the one of the at least one section of theoptical fiber located along another end third of the one of the at leastone mandrel in the lengthwise dimension in response to third componentsof the return optical signal redirected by the third mandrel zoneshaving lower noise components than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the other end third of the one of the at least one mandrel, todetermine respective phases of the first, second, and third phases ofthe first, second, and third components of the return optical signal,and to determine that the acoustic signal is incident on the one of theat least one mandrel in response to a difference between a sum of thefirst and second phases and a sum of the third phases.

Example 69 includes the system of any of Examples 53-68, wherein: thefirst mandrel zone is located along an end third of the one of the atleast one mandrel in a lengthwise dimension; and the electronic circuitis further configured: to select the first mandrel zone and at least onesecond mandrel zone located along the end third of the one of the atleast one mandrel in response to the first component of the returnoptical signal and at least one second component of the return opticalsignal respectively redirected by the at least one second mandrel zonehaving lower noise components than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the end third of the one of the at least one mandrel, to selectthird mandrel zones of the one of the at least one section of theoptical fiber located along another end third of the one of the at leastone mandrel in the lengthwise dimension in response to third componentsof the return optical signal redirected by the third mandrel zoneshaving lower noise components than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the other end third of the one of the at least one mandrel, todetermine respective phases of the first, second, and third phases ofthe first, second, and third components of the return optical signal,and to determine that the acoustic signal is incident on the one of theat least one mandrel in response to a difference between an average ofthe first and second phases and an average of the third phases.

Example 70 includes the system of any of Examples 53-69, wherein: thefirst mandrel zone is located along an end third of the one of the atleast one mandrel in a lengthwise dimension; and the electronic circuitis further configured: to select the first mandrel zone and at least onesecond mandrel zone located along the end third of the one of the atleast one mandrel in response to the first component of the returnoptical signal and at least one second component of the return opticalsignal respectively redirected by the at least one second mandrel zonehaving higher optical powers than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the end third of the one of the at least one mandrel, to selectthird mandrel zones of the one of the at least one section of theoptical fiber located along another end third of the one of the at leastone mandrel in the lengthwise dimension in response to third componentsof the return optical signal redirected by the third mandrel zoneshaving higher optical powers than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the other end third of the one of the at least one mandrel, todetermine respective phases of the first, second, and third phases ofthe first, second, and third components of the return optical signal,and to determine that the acoustic signal is incident on the one of theat least one mandrel in response to a combination of the first, second,and third phases.

Example 71 includes the system of any of Examples 53-70, wherein: thefirst mandrel zone is located along an end third of the one of the atleast one mandrel in a lengthwise dimension; and the electronic circuitis further configured: to select the first mandrel zone and at least onesecond mandrel zone located along the end third of the one of the atleast one mandrel in response to the first component of the returnoptical signal and at least one second component of the return opticalsignal respectively redirected by the at least one second mandrel zonehaving higher optical powers than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the end third of the one of the at least one mandrel, to selectthird mandrel zones of the one of the at least one section of theoptical fiber located along another end third of the one of the at leastone mandrel in the lengthwise dimension in response to third componentsof the return optical signal redirected by the third mandrel zoneshaving higher optical powers than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the other end third of the one of the at least one mandrel, todetermine respective phases of the first, second, and third phases ofthe first, second, and third components of the return optical signal,and to determine that the acoustic signal is incident on the one of theat least one mandrel in response to a difference between a sum of thefirst and second phases and a sum of the third phases.

Example 72 includes the system of any of Examples 53-71, wherein: thefirst mandrel zone is located along an end third of the one of the atleast one mandrel in a lengthwise dimension; and the electronic circuitis further configured: to select the first mandrel zone and at least onesecond mandrel zone located along the end third of the one of the atleast one mandrel in response to the first component of the returnoptical signal and at least one second component of the return opticalsignal respectively redirected by the at least one second mandrel zonehaving higher optical powers than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the end third of the one of the at least one mandrel, to selectthird mandrel zones of the one of the at least one section of theoptical fiber located along another end third of the one of the at leastone mandrel in the lengthwise dimension in response to third componentsof the return optical signal redirected by the third mandrel zoneshaving higher optical powers than at least one other component of thereturn optical signal each redirected by a respective other mandrel zoneof the one of the at least one section of the optical fiber locatedalong the other end third of the one of the at least one mandrel, todetermine respective phases of the first, second, and third phases ofthe first, second, and third components of the return optical signal,and to determine that the acoustic signal is incident on the one of theat least one mandrel in response to a difference between an average ofthe first and second phases and an average of the third phases.

Example 73 includes the system of any of Examples 53-72, furthercomprising the optical-fiber assembly.

Example 74 includes a method, comprising: directing a source opticalsignal into an end of an optical-fiber assembly that includes an opticalfiber having at least one section each wrapped multiple turns around arespective one of at least one mandrel and each including respectivemandrel zones; receiving, from the end of the optical-fiber assembly, areturn optical signal; selecting a first mandrel zone of one of the atleast one section of the optical fiber in response to a first componentof the return optical signal redirected by the first mandrel zone; anddetermining that an acoustic signal is incident on the one of the atleast one mandrel around which the one of the at least one section ofthe optical fiber is wrapped in response to the first component of thereturn optical signal.

Example 75 includes the method of Example 74, further comprisingselecting the first mandrel zone in response to the first component ofthe return optical signal having a characteristic that is closer to atarget than at least one other component of the return optical signaleach redirected by a respective other mandrel zone of the one of the atleast one section of the optical fiber.

Example 76 includes the method of any of Examples 74-75, furthercomprising selecting the first mandrel zone in response to the firstcomponent of the return optical signal having a lower noise componentthan at least one other component of the return optical signal eachredirected by a respective other mandrel zone of the one of the at leastone section of the optical fiber.

Example 77 includes the method of any of Examples 74-76, furthercomprising selecting the first mandrel zone in response to the firstcomponent of the return optical signal having a higher optical powerthan at least one other component of the return optical signal eachredirected by a respective other mandrel zone of the one of the at leastone section of the optical fiber.

Example 78 includes the method of any of Examples 74-77, furthercomprising determining a frequency of the acoustic signal in response tothe first component of the return optical signal.

Example 79 includes the method of any of Examples 74-78, furthercomprising determining a strength of the acoustic signal in response tothe first component of the return optical signal.

Example 80 includes the method of any of Examples 74-79, furthercomprising determining an amplitude of the acoustic signal in responseto the first component of the return optical signal.

Example 81 includes the method of any of Examples 74-80, furthercomprising determining a power of the acoustic signal in response to thefirst component of the return optical signal.

Example 82 includes the method of any of Examples 74-81, furthercomprising: selecting a second mandrel zone of the one of the at leastone section of the optical fiber in response to a second component ofthe return optical signal redirected by the second mandrel zone; anddetermining that the acoustic signal is incident on the one of the atleast one mandrel in response to the second component of the returnoptical signal.

Example 83 includes the method of any of Examples 74-82, furthercomprising: wherein the first mandrel zone is located along a half ofthe one of the at least one mandrel in a lengthwise dimension; selectinga second mandrel zone of the one of the at least one section of theoptical fiber in response to a second component of the return opticalsignal redirected by the second mandrel zone, the second mandrel zone islocated along another half of the one of the at least one mandrel in thelengthwise dimension; determining that the acoustic signal is incidenton the one of the at least one mandrel in response to the first andsecond components of the return optical signal.

Example 84 includes the method of any of Examples 74-83, furthercomprising: wherein the first mandrel zone is located along a half ofthe one of the at least one mandrel in a lengthwise dimension; selectinga second mandrel zone of the one of the at least one section of theoptical fiber in response to a second component of the return opticalsignal redirected by the second mandrel zone, the second mandrel zone islocated along another half of the one of the at least one mandrel in thelengthwise dimension; determining respective first and second values ofa characteristic of the first and second components of the returnoptical signal; and determining that the acoustic signal is incident onthe one of the at least one mandrel in response to a combination of thefirst and second values.

Example 85 includes the method of any of Examples 74-84, furthercomprising: wherein the first mandrel zone is located along a half ofthe one of the at least one mandrel in a lengthwise dimension; selectinga second mandrel zone of the one of the at least one section of theoptical fiber in response to a second component of the return opticalsignal redirected by the second mandrel zone, the second mandrel zone islocated along another half of the one of the at least one mandrel in thelengthwise dimension; determining respective first and second phases ofthe first and second components of the return optical signal; anddetermining that the acoustic signal is incident on the one of the atleast one mandrel in response to a difference between the first andsecond phases.

Example 86 includes the method of any of Examples 74-85, furthercomprising: wherein the first mandrel zone is located along a half ofthe one of the at least one mandrel in a lengthwise dimension; selectingthe first mandrel zone in response to the first component of the returnoptical signal having a lower noise component than at least one othercomponent of the return optical signal each redirected by a respectiveother mandrel zone of the one of the at least one section of the opticalfiber located along the half of the one of the at least one mandrel;selecting a second mandrel zone of the one of the at least one sectionof the optical fiber located along another half of the one of the atleast one mandrel in the lengthwise dimension in response to a secondcomponent of the return optical signal redirected by the second mandrelzone having a lower noise component than at least one other component ofthe return optical signal each redirected by a respective other mandrelzone of the one of the at least one section of the optical fiber locatedalong the other half of the one of the at least one mandrel; determiningrespective first and second phases of the first and second components ofthe return optical signal; and determining that the acoustic signal isincident on the one of the at least one mandrel in response to adifference between the first and second phases.

Example 87 includes the method of any of Examples 74-86, furthercomprising: wherein the first mandrel zone is located along a half ofthe one of the at least one mandrel in a lengthwise dimension; selectingthe first mandrel zone in response to the first component of the returnoptical signal having a higher optical power than at least one othercomponent of the return optical signal each redirected by a respectiveother mandrel zone of the one of the at least one section of the opticalfiber located along the half of the one of the at least one mandrel;selecting a second mandrel zone of the one of the at least one sectionof the optical fiber located along another half of the one of the atleast one mandrel in the lengthwise dimension in response to a secondcomponent of the return optical signal redirected by the second mandrelzone having a higher optical power than at least one other component ofthe return optical signal each redirected by a respective other mandrelzone of the one of the at least one section of the optical fiber locatedalong the other half of the one of the at least one mandrel; determiningrespective first and second phases of the first and second components ofthe return optical signal; and determining that the acoustic signal isincident on the one of the at least one mandrel in response to adifference between the first and second phases.

Example 88 includes the method of any of Examples 74-87, furthercomprising: wherein the first mandrel zone is located along an end thirdof the one of the at least one mandrel in a lengthwise dimension;selecting the first mandrel zone and at least one second mandrel zonelocated along the end third of the one of the at least one mandrel inresponse to the first component of the return optical signal and atleast one second component of the return optical signal respectivelyredirected by the at least one second mandrel zone having lower noisecomponents than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the endthird of the one of the at least one mandrel; selecting third mandrelzones of the one of the at least one section of the optical fiberlocated along another end third of the one of the at least one mandrelin the lengthwise dimension in response to third components of thereturn optical signal redirected by the third mandrel zones having lowernoise components than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the otherend third of the one of the at least one mandrel; determining respectivephases of the first, second, and third phases of the first, second, andthird components of the return optical signal; and determining that theacoustic signal is incident on the one of the at least one mandrel inresponse to a combination of the first, second, and third phases.

Example 89 includes the method of any of Examples 74-88, furthercomprising: wherein the first mandrel zone is located along an end thirdof the one of the at least one mandrel in a lengthwise dimension;selecting the first mandrel zone and at least one second mandrel zonelocated along the end third of the one of the at least one mandrel inresponse to the first component of the return optical signal and atleast one second component of the return optical signal respectivelyredirected by the at least one second mandrel zone having lower noisecomponents than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the endthird of the one of the at least one mandrel; selecting third mandrelzones of the one of the at least one section of the optical fiberlocated along another end third of the one of the at least one mandrelin the lengthwise dimension in response to third components of thereturn optical signal redirected by the third mandrel zones having lowernoise components than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the otherend third of the one of the at least one mandrel; determining respectivephases of the first, second, and third phases of the first, second, andthird components of the return optical signal; and determining that theacoustic signal is incident on the one of the at least one mandrel inresponse to a difference between a sum of the first and second phasesand a sum of the third phases.

Example 90 includes the method of any of Examples 74-89, furthercomprising: wherein the first mandrel zone is located along an end thirdof the one of the at least one mandrel in a lengthwise dimension;selecting the first mandrel zone and at least one second mandrel zonelocated along the end third of the one of the at least one mandrel inresponse to the first component of the return optical signal and atleast one second component of the return optical signal respectivelyredirected by the at least one second mandrel zone having lower noisecomponents than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the endthird of the one of the at least one mandrel; selecting third mandrelzones of the one of the at least one section of the optical fiberlocated along another end third of the one of the at least one mandrelin the lengthwise dimension in response to third components of thereturn optical signal redirected by the third mandrel zones having lowernoise components than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the otherend third of the one of the at least one mandrel; determining respectivephases of the first, second, and third phases of the first, second, andthird components of the return optical signal; and determining that theacoustic signal is incident on the one of the at least one mandrel inresponse to a difference between an average of the first and secondphases and an average of the third phases.

Example 91 includes the method of any of Examples 74-90, furthercomprising: wherein the first mandrel zone is located along an end thirdof the one of the at least one mandrel in a lengthwise dimension;selecting the first mandrel zone and at least one second mandrel zonelocated along the end third of the one of the at least one mandrel inresponse to the first component of the return optical signal and atleast one second component of the return optical signal respectivelyredirected by the at least one second mandrel zone having higher opticalpowers than at least one other component of the return optical signaleach redirected by a respective other mandrel zone of the one of the atleast one section of the optical fiber located along the end third ofthe one of the at least one mandrel; selecting third mandrel zones ofthe one of the at least one section of the optical fiber located alonganother end third of the one of the at least one mandrel in thelengthwise dimension in response to third components of the returnoptical signal redirected by the third mandrel zones having higheroptical powers than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the otherend third of the one of the at least one mandrel; determining respectivephases of the first, second, and third phases of the first, second, andthird components of the return optical signal; determining that theacoustic signal is incident on the one of the at least one mandrel inresponse to a combination of the first, second, and third phases.

Example 92 includes the method of any of Examples 74-91, furthercomprising: wherein the first mandrel zone is located along an end thirdof the one of the at least one mandrel in a lengthwise dimension;selecting the first mandrel zone and at least one second mandrel zonelocated along the end third of the one of the at least one mandrel inresponse to the first component of the return optical signal and atleast one second component of the return optical signal respectivelyredirected by the at least one second mandrel zone having higher opticalpowers than at least one other component of the return optical signaleach redirected by a respective other mandrel zone of the one of the atleast one section of the optical fiber located along the end third ofthe one of the at least one mandrel; selecting third mandrel zones ofthe one of the at least one section of the optical fiber located alonganother end third of the one of the at least one mandrel in thelengthwise dimension in response to third components of the returnoptical signal redirected by the third mandrel zones having higheroptical powers than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the otherend third of the one of the at least one mandrel; determining respectivephases of the first, second, and third phases of the first, second, andthird components of the return optical signal; and determining that theacoustic signal is incident on the one of the at least one mandrel inresponse to a difference between a sum of the first and second phasesand a sum of the third phases.

Example 93 includes the method of any of Examples 74-92, furthercomprising: wherein the first mandrel zone is located along an end thirdof the one of the at least one mandrel in a lengthwise dimension;selecting the first mandrel zone and at least one second mandrel zonelocated along the end third of the one of the at least one mandrel inresponse to the first component of the return optical signal and atleast one second component of the return optical signal respectivelyredirected by the at least one second mandrel zone having higher opticalpowers than at least one other component of the return optical signaleach redirected by a respective other mandrel zone of the one of the atleast one section of the optical fiber located along the end third ofthe one of the at least one mandrel; selecting third mandrel zones ofthe one of the at least one section of the optical fiber located alonganother end third of the one of the at least one mandrel in thelengthwise dimension in response to third components of the returnoptical signal redirected by the third mandrel zones having higheroptical powers than at least one other component of the return opticalsignal each redirected by a respective other mandrel zone of the one ofthe at least one section of the optical fiber located along the otherend third of the one of the at least one mandrel; determining respectivephases of the first, second, and third phases of the first, second, andthird components of the return optical signal; and determining that theacoustic signal is incident on the one of the at least one mandrel inresponse to a difference between an average of the first and secondphases and an average of the third phases.

Example 94 includes a computer-readable medium storing data that whenexecuted by, or used to configure, an electronic circuit causes theelectronic circuit, or one or more other electronic circuits under thecontrol of the electronic circuit: to source an optical beam into an endof an optical fiber that extends through an optical-fiber path of arespective first end cap of each at least one mandrel, that is woundabout a respective outer conduit of each of the at least one mandrel,and that extends through an output-fiber path of a respective second endcap of each of the at least one mandrel fiber; to receive a redirectedoptical beam from the end of the optical fiber; and to detect anacoustic signal incident on at least one of the at least one mandrel inresponse to the redirected optical beam.

Example 95 includes a computer-readable medium storing data that whenexecuted by, or used to configure, an electronic circuit causes theelectronic circuit, or one or more other electronic circuits under thecontrol of the electronic circuit: to direct a source optical signalinto an end of an optical-fiber assembly that includes an optical fiberhaving at least one section each wrapped multiple turns around arespective one of at least one mandrel and each including respectivemandrel zones; to receive, from the end of the optical-fiber assembly, areturn optical signal; to select a first mandrel zone of one of the atleast one section of the optical fiber in response to a first componentof the return optical signal redirected by the first mandrel zone; andto determine that an acoustic signal is incident on the one of the atleast one mandrel around which the one of the at least one section ofthe optical fiber is wrapped in response to the first component of thereturn optical signal.

1. A mandrel, comprising: a connector having first and second alignedopenings and a third opening between the first and second openings; anda form coupled to the connector at the third opening, having an outersurface, a cavity, an end, and fourth and fifth openings between thecavity and the outer surface, and configured to receive an optical fiberthat extends into the first opening of the connector, through the thirdopening of the connector, into the cavity at the end of the form, andthrough the fourth opening of the form, that forms one or more turnsaround the outer surface of the form, and that extends through the fifthopening of the form into the cavity, out from the cavity at the end,into the third opening of the connector, and out from the second openingof the connector.